[pull] master from google-deepmind:master - #64

pad_to_size hardcoded pad_width as ((top, bottom), (left, right), (0, 0))
which assumes channels-last (HWC) layout regardless of the channel_axis
argument. For channels-first (CHW) images, this causes the padding to be
applied along the channel dimension and leaves the width dimension unpadded,
producing a wrong output shape (e.g. (5, 6, 4) instead of (3, 6, 6)).

Fix: branch on _channels_last(image, channel_axis) and produce
((0, 0), (top, bottom), (left, right)) for CHW layout, consistent
with how every other function in this module handles channel_axis.

Add pad_to_size_test.py covering:
- HWC baseline (channel_axis=-1): shape is preserved correctly.
- CHW shape (channel_axis=0): must be (C, H', W') not (C+pad, H', W).
- CHW values (channel_axis=0): original pixels centered, border is zero.
- Batched CHW (channel_axis=1): must be (B, C, H', W').

Address review feedback on PR #112:
- Move the channels-first pad_to_size regression tests out of the
  standalone pad_to_size_test.py and into the existing TestCustom class
  in augment_test.py, matching that file's conventions.
- Drop the explanatory comments, consistent with the surrounding tests.

Delete dm_pix/_src/pad_to_size_test.py.

PiperOrigin-RevId: 925200395

PiperOrigin-RevId: 925269255