Add support for global layer options - #563

This allows to configure model layers at Axon-compilation time. This is useful if we want to make some compilation-time decisions when calling the model, without building a different model altogether.

API-wise the options are passed as

Axon.build(model, global_layer_options: [output_hidden_states: true])

Those options are merged into layer options, but only when declared explicitly, like so:

Axon.layer(
  fn hidden_state,  opts ->
    if opts[:output_hidden_states] do
      ...
    else
      ...
    end
  end,
  [input],
  global_options: [:output_hidden_states]
)

We could pass them nested too and access as opts[:global_options][:output_hidden_states], but this would break all existing layers that have options allowlist using keyword!. I actually think that merging them in a flat manner is better, because they can be validated with keyword! in the exact same way.

Naming suggestions are welcome, for example I was considering :global_layer_options -> :layer_options and :global_options -> :expose_options, though not sure if "expose" is clear enough.