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# Copyright 2025 The HuggingFace Team. All rights reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import functools
import math
import flax.linen as nn
import jax
import jax.numpy as jnp
from ..utils import logging
logger = logging.get_logger(__name__)
def _query_chunk_attention(query, key, value, precision, key_chunk_size: int = 4096):
"""Multi-head dot product attention with a limited number of queries."""
num_kv, num_heads, k_features = key.shape[-3:]
v_features = value.shape[-1]
key_chunk_size = min(key_chunk_size, num_kv)
query = query / jnp.sqrt(k_features)
@functools.partial(jax.checkpoint, prevent_cse=False)
def summarize_chunk(query, key, value):
attn_weights = jnp.einsum("...qhd,...khd->...qhk", query, key, precision=precision)
max_score = jnp.max(attn_weights, axis=-1, keepdims=True)
max_score = jax.lax.stop_gradient(max_score)
exp_weights = jnp.exp(attn_weights - max_score)
exp_values = jnp.einsum("...vhf,...qhv->...qhf", value, exp_weights, precision=precision)
max_score = jnp.einsum("...qhk->...qh", max_score)
return (exp_values, exp_weights.sum(axis=-1), max_score)
def chunk_scanner(chunk_idx):
# julienne key array
key_chunk = jax.lax.dynamic_slice(
operand=key,
start_indices=[0] * (key.ndim - 3) + [chunk_idx, 0, 0], # [...,k,h,d]
slice_sizes=list(key.shape[:-3]) + [key_chunk_size, num_heads, k_features], # [...,k,h,d]
)
# julienne value array
value_chunk = jax.lax.dynamic_slice(
operand=value,
start_indices=[0] * (value.ndim - 3) + [chunk_idx, 0, 0], # [...,v,h,d]
slice_sizes=list(value.shape[:-3]) + [key_chunk_size, num_heads, v_features], # [...,v,h,d]
)
return summarize_chunk(query, key_chunk, value_chunk)
chunk_values, chunk_weights, chunk_max = jax.lax.map(f=chunk_scanner, xs=jnp.arange(0, num_kv, key_chunk_size))
global_max = jnp.max(chunk_max, axis=0, keepdims=True)
max_diffs = jnp.exp(chunk_max - global_max)
chunk_values *= jnp.expand_dims(max_diffs, axis=-1)
chunk_weights *= max_diffs
all_values = chunk_values.sum(axis=0)
all_weights = jnp.expand_dims(chunk_weights, -1).sum(axis=0)
return all_values / all_weights
def jax_memory_efficient_attention(
query, key, value, precision=jax.lax.Precision.HIGHEST, query_chunk_size: int = 1024, key_chunk_size: int = 4096
):
r"""
Flax Memory-efficient multi-head dot product attention. https://huggingface.co/papers/2112.05682v2
https://github.com/AminRezaei0x443/memory-efficient-attention
Args:
query (`jnp.ndarray`): (batch..., query_length, head, query_key_depth_per_head)
key (`jnp.ndarray`): (batch..., key_value_length, head, query_key_depth_per_head)
value (`jnp.ndarray`): (batch..., key_value_length, head, value_depth_per_head)
precision (`jax.lax.Precision`, *optional*, defaults to `jax.lax.Precision.HIGHEST`):
numerical precision for computation
query_chunk_size (`int`, *optional*, defaults to 1024):
chunk size to divide query array value must divide query_length equally without remainder
key_chunk_size (`int`, *optional*, defaults to 4096):
chunk size to divide key and value array value must divide key_value_length equally without remainder
Returns:
(`jnp.ndarray`) with shape of (batch..., query_length, head, value_depth_per_head)
"""
num_q, num_heads, q_features = query.shape[-3:]
def chunk_scanner(chunk_idx, _):
# julienne query array
query_chunk = jax.lax.dynamic_slice(
operand=query,
start_indices=([0] * (query.ndim - 3)) + [chunk_idx, 0, 0], # [...,q,h,d]
slice_sizes=list(query.shape[:-3]) + [min(query_chunk_size, num_q), num_heads, q_features], # [...,q,h,d]
)
return (
chunk_idx + query_chunk_size, # unused ignore it
_query_chunk_attention(
query=query_chunk, key=key, value=value, precision=precision, key_chunk_size=key_chunk_size
),
)
_, res = jax.lax.scan(
f=chunk_scanner,
init=0,
xs=None,
length=math.ceil(num_q / query_chunk_size), # start counter # stop counter
)
return jnp.concatenate(res, axis=-3) # fuse the chunked result back
class FlaxAttention(nn.Module):
r"""
A Flax multi-head attention module as described in: https://huggingface.co/papers/1706.03762
Parameters:
query_dim (:obj:`int`):
Input hidden states dimension
heads (:obj:`int`, *optional*, defaults to 8):
Number of heads
dim_head (:obj:`int`, *optional*, defaults to 64):
Hidden states dimension inside each head
dropout (:obj:`float`, *optional*, defaults to 0.0):
Dropout rate
use_memory_efficient_attention (`bool`, *optional*, defaults to `False`):
enable memory efficient attention https://huggingface.co/papers/2112.05682
split_head_dim (`bool`, *optional*, defaults to `False`):
Whether to split the head dimension into a new axis for the self-attention computation. In most cases,
enabling this flag should speed up the computation for Stable Diffusion 2.x and Stable Diffusion XL.
dtype (:obj:`jnp.dtype`, *optional*, defaults to jnp.float32):
Parameters `dtype`
"""
query_dim: int
heads: int = 8
dim_head: int = 64
dropout: float = 0.0
use_memory_efficient_attention: bool = False
split_head_dim: bool = False
dtype: jnp.dtype = jnp.float32
def setup(self):
logger.warning(
"Flax classes are deprecated and will be removed in Diffusers v1.0.0. We "
"recommend migrating to PyTorch classes or pinning your version of Diffusers."
)
inner_dim = self.dim_head * self.heads
self.scale = self.dim_head**-0.5
# Weights were exported with old names {to_q, to_k, to_v, to_out}
self.query = nn.Dense(inner_dim, use_bias=False, dtype=self.dtype, name="to_q")
self.key = nn.Dense(inner_dim, use_bias=False, dtype=self.dtype, name="to_k")
self.value = nn.Dense(inner_dim, use_bias=False, dtype=self.dtype, name="to_v")
self.proj_attn = nn.Dense(self.query_dim, dtype=self.dtype, name="to_out_0")
self.dropout_layer = nn.Dropout(rate=self.dropout)
def reshape_heads_to_batch_dim(self, tensor):
batch_size, seq_len, dim = tensor.shape
head_size = self.heads
tensor = tensor.reshape(batch_size, seq_len, head_size, dim // head_size)
tensor = jnp.transpose(tensor, (0, 2, 1, 3))
tensor = tensor.reshape(batch_size * head_size, seq_len, dim // head_size)
return tensor
def reshape_batch_dim_to_heads(self, tensor):
batch_size, seq_len, dim = tensor.shape
head_size = self.heads
tensor = tensor.reshape(batch_size // head_size, head_size, seq_len, dim)
tensor = jnp.transpose(tensor, (0, 2, 1, 3))
tensor = tensor.reshape(batch_size // head_size, seq_len, dim * head_size)
return tensor
def __call__(self, hidden_states, context=None, deterministic=True):
context = hidden_states if context is None else context
query_proj = self.query(hidden_states)
key_proj = self.key(context)
value_proj = self.value(context)
if self.split_head_dim:
b = hidden_states.shape[0]
query_states = jnp.reshape(query_proj, (b, -1, self.heads, self.dim_head))
key_states = jnp.reshape(key_proj, (b, -1, self.heads, self.dim_head))
value_states = jnp.reshape(value_proj, (b, -1, self.heads, self.dim_head))
else:
query_states = self.reshape_heads_to_batch_dim(query_proj)
key_states = self.reshape_heads_to_batch_dim(key_proj)
value_states = self.reshape_heads_to_batch_dim(value_proj)
if self.use_memory_efficient_attention:
query_states = query_states.transpose(1, 0, 2)
key_states = key_states.transpose(1, 0, 2)
value_states = value_states.transpose(1, 0, 2)
# this if statement create a chunk size for each layer of the unet
# the chunk size is equal to the query_length dimension of the deepest layer of the unet
flatten_latent_dim = query_states.shape[-3]
if flatten_latent_dim % 64 == 0:
query_chunk_size = int(flatten_latent_dim / 64)
elif flatten_latent_dim % 16 == 0:
query_chunk_size = int(flatten_latent_dim / 16)
elif flatten_latent_dim % 4 == 0:
query_chunk_size = int(flatten_latent_dim / 4)
else:
query_chunk_size = int(flatten_latent_dim)
hidden_states = jax_memory_efficient_attention(
query_states, key_states, value_states, query_chunk_size=query_chunk_size, key_chunk_size=4096 * 4
)
hidden_states = hidden_states.transpose(1, 0, 2)
hidden_states = self.reshape_batch_dim_to_heads(hidden_states)
else:
# compute attentions
if self.split_head_dim:
attention_scores = jnp.einsum("b t n h, b f n h -> b n f t", key_states, query_states)
else:
attention_scores = jnp.einsum("b i d, b j d->b i j", query_states, key_states)
attention_scores = attention_scores * self.scale
attention_probs = nn.softmax(attention_scores, axis=-1 if self.split_head_dim else 2)
# attend to values
if self.split_head_dim:
hidden_states = jnp.einsum("b n f t, b t n h -> b f n h", attention_probs, value_states)
b = hidden_states.shape[0]
hidden_states = jnp.reshape(hidden_states, (b, -1, self.heads * self.dim_head))
else:
hidden_states = jnp.einsum("b i j, b j d -> b i d", attention_probs, value_states)
hidden_states = self.reshape_batch_dim_to_heads(hidden_states)
hidden_states = self.proj_attn(hidden_states)
return self.dropout_layer(hidden_states, deterministic=deterministic)
class FlaxBasicTransformerBlock(nn.Module):
r"""
A Flax transformer block layer with `GLU` (Gated Linear Unit) activation function as described in:
https://huggingface.co/papers/1706.03762
Parameters:
dim (:obj:`int`):
Inner hidden states dimension
n_heads (:obj:`int`):
Number of heads
d_head (:obj:`int`):
Hidden states dimension inside each head
dropout (:obj:`float`, *optional*, defaults to 0.0):
Dropout rate
only_cross_attention (`bool`, defaults to `False`):
Whether to only apply cross attention.
dtype (:obj:`jnp.dtype`, *optional*, defaults to jnp.float32):
Parameters `dtype`
use_memory_efficient_attention (`bool`, *optional*, defaults to `False`):
enable memory efficient attention https://huggingface.co/papers/2112.05682
split_head_dim (`bool`, *optional*, defaults to `False`):
Whether to split the head dimension into a new axis for the self-attention computation. In most cases,
enabling this flag should speed up the computation for Stable Diffusion 2.x and Stable Diffusion XL.
"""
dim: int
n_heads: int
d_head: int
dropout: float = 0.0
only_cross_attention: bool = False
dtype: jnp.dtype = jnp.float32
use_memory_efficient_attention: bool = False
split_head_dim: bool = False
def setup(self):
logger.warning(
"Flax classes are deprecated and will be removed in Diffusers v1.0.0. We "
"recommend migrating to PyTorch classes or pinning your version of Diffusers."
)
# self attention (or cross_attention if only_cross_attention is True)
self.attn1 = FlaxAttention(
self.dim,
self.n_heads,
self.d_head,
self.dropout,
self.use_memory_efficient_attention,
self.split_head_dim,
dtype=self.dtype,
)
# cross attention
self.attn2 = FlaxAttention(
self.dim,
self.n_heads,
self.d_head,
self.dropout,
self.use_memory_efficient_attention,
self.split_head_dim,
dtype=self.dtype,
)
self.ff = FlaxFeedForward(dim=self.dim, dropout=self.dropout, dtype=self.dtype)
self.norm1 = nn.LayerNorm(epsilon=1e-5, dtype=self.dtype)
self.norm2 = nn.LayerNorm(epsilon=1e-5, dtype=self.dtype)
self.norm3 = nn.LayerNorm(epsilon=1e-5, dtype=self.dtype)
self.dropout_layer = nn.Dropout(rate=self.dropout)
def __call__(self, hidden_states, context, deterministic=True):
# self attention
residual = hidden_states
if self.only_cross_attention:
hidden_states = self.attn1(self.norm1(hidden_states), context, deterministic=deterministic)
else:
hidden_states = self.attn1(self.norm1(hidden_states), deterministic=deterministic)
hidden_states = hidden_states + residual
# cross attention
residual = hidden_states
hidden_states = self.attn2(self.norm2(hidden_states), context, deterministic=deterministic)
hidden_states = hidden_states + residual
# feed forward
residual = hidden_states
hidden_states = self.ff(self.norm3(hidden_states), deterministic=deterministic)
hidden_states = hidden_states + residual
return self.dropout_layer(hidden_states, deterministic=deterministic)
class FlaxTransformer2DModel(nn.Module):
r"""
A Spatial Transformer layer with Gated Linear Unit (GLU) activation function as described in:
https://huggingface.co/papers/1506.02025
Parameters:
in_channels (:obj:`int`):
Input number of channels
n_heads (:obj:`int`):
Number of heads
d_head (:obj:`int`):
Hidden states dimension inside each head
depth (:obj:`int`, *optional*, defaults to 1):
Number of transformers block
dropout (:obj:`float`, *optional*, defaults to 0.0):
Dropout rate
use_linear_projection (`bool`, defaults to `False`): tbd
only_cross_attention (`bool`, defaults to `False`): tbd
dtype (:obj:`jnp.dtype`, *optional*, defaults to jnp.float32):
Parameters `dtype`
use_memory_efficient_attention (`bool`, *optional*, defaults to `False`):
enable memory efficient attention https://huggingface.co/papers/2112.05682
split_head_dim (`bool`, *optional*, defaults to `False`):
Whether to split the head dimension into a new axis for the self-attention computation. In most cases,
enabling this flag should speed up the computation for Stable Diffusion 2.x and Stable Diffusion XL.
"""
in_channels: int
n_heads: int
d_head: int
depth: int = 1
dropout: float = 0.0
use_linear_projection: bool = False
only_cross_attention: bool = False
dtype: jnp.dtype = jnp.float32
use_memory_efficient_attention: bool = False
split_head_dim: bool = False
def setup(self):
logger.warning(
"Flax classes are deprecated and will be removed in Diffusers v1.0.0. We "
"recommend migrating to PyTorch classes or pinning your version of Diffusers."
)
self.norm = nn.GroupNorm(num_groups=32, epsilon=1e-5)
inner_dim = self.n_heads * self.d_head
if self.use_linear_projection:
self.proj_in = nn.Dense(inner_dim, dtype=self.dtype)
else:
self.proj_in = nn.Conv(
inner_dim,
kernel_size=(1, 1),
strides=(1, 1),
padding="VALID",
dtype=self.dtype,
)
self.transformer_blocks = [
FlaxBasicTransformerBlock(
inner_dim,
self.n_heads,
self.d_head,
dropout=self.dropout,
only_cross_attention=self.only_cross_attention,
dtype=self.dtype,
use_memory_efficient_attention=self.use_memory_efficient_attention,
split_head_dim=self.split_head_dim,
)
for _ in range(self.depth)
]
if self.use_linear_projection:
self.proj_out = nn.Dense(inner_dim, dtype=self.dtype)
else:
self.proj_out = nn.Conv(
inner_dim,
kernel_size=(1, 1),
strides=(1, 1),
padding="VALID",
dtype=self.dtype,
)
self.dropout_layer = nn.Dropout(rate=self.dropout)
def __call__(self, hidden_states, context, deterministic=True):
batch, height, width, channels = hidden_states.shape
residual = hidden_states
hidden_states = self.norm(hidden_states)
if self.use_linear_projection:
hidden_states = hidden_states.reshape(batch, height * width, channels)
hidden_states = self.proj_in(hidden_states)
else:
hidden_states = self.proj_in(hidden_states)
hidden_states = hidden_states.reshape(batch, height * width, channels)
for transformer_block in self.transformer_blocks:
hidden_states = transformer_block(hidden_states, context, deterministic=deterministic)
if self.use_linear_projection:
hidden_states = self.proj_out(hidden_states)
hidden_states = hidden_states.reshape(batch, height, width, channels)
else:
hidden_states = hidden_states.reshape(batch, height, width, channels)
hidden_states = self.proj_out(hidden_states)
hidden_states = hidden_states + residual
return self.dropout_layer(hidden_states, deterministic=deterministic)
class FlaxFeedForward(nn.Module):
r"""
Flax module that encapsulates two Linear layers separated by a non-linearity. It is the counterpart of PyTorch's
[`FeedForward`] class, with the following simplifications:
- The activation function is currently hardcoded to a gated linear unit from:
https://huggingface.co/papers/2002.05202
- `dim_out` is equal to `dim`.
- The number of hidden dimensions is hardcoded to `dim * 4` in [`FlaxGELU`].
Parameters:
dim (:obj:`int`):
Inner hidden states dimension
dropout (:obj:`float`, *optional*, defaults to 0.0):
Dropout rate
dtype (:obj:`jnp.dtype`, *optional*, defaults to jnp.float32):
Parameters `dtype`
"""
dim: int
dropout: float = 0.0
dtype: jnp.dtype = jnp.float32
def setup(self):
logger.warning(
"Flax classes are deprecated and will be removed in Diffusers v1.0.0. We "
"recommend migrating to PyTorch classes or pinning your version of Diffusers."
)
# The second linear layer needs to be called
# net_2 for now to match the index of the Sequential layer
self.net_0 = FlaxGEGLU(self.dim, self.dropout, self.dtype)
self.net_2 = nn.Dense(self.dim, dtype=self.dtype)
def __call__(self, hidden_states, deterministic=True):
hidden_states = self.net_0(hidden_states, deterministic=deterministic)
hidden_states = self.net_2(hidden_states)
return hidden_states
class FlaxGEGLU(nn.Module):
r"""
Flax implementation of a Linear layer followed by the variant of the gated linear unit activation function from
https://huggingface.co/papers/2002.05202.
Parameters:
dim (:obj:`int`):
Input hidden states dimension
dropout (:obj:`float`, *optional*, defaults to 0.0):
Dropout rate
dtype (:obj:`jnp.dtype`, *optional*, defaults to jnp.float32):
Parameters `dtype`
"""
dim: int
dropout: float = 0.0
dtype: jnp.dtype = jnp.float32
def setup(self):
logger.warning(
"Flax classes are deprecated and will be removed in Diffusers v1.0.0. We "
"recommend migrating to PyTorch classes or pinning your version of Diffusers."
)
inner_dim = self.dim * 4
self.proj = nn.Dense(inner_dim * 2, dtype=self.dtype)
self.dropout_layer = nn.Dropout(rate=self.dropout)
def __call__(self, hidden_states, deterministic=True):
hidden_states = self.proj(hidden_states)
hidden_linear, hidden_gelu = jnp.split(hidden_states, 2, axis=2)
return self.dropout_layer(hidden_linear * nn.gelu(hidden_gelu), deterministic=deterministic)