set expand option as default for @layer

[CarloLucibello]

This introduces the new show option :noexpand for @layer and sets the default to :expand instead.

Fix #2531

[CarloLucibello]

unrelated change: with v0.15 we don't need to define gpu_backend!, we can just reexport the one from MLDataDevices

[mcabbott]

These break, and probably others. Before @layer there was lots of special-case code to catch all of them.

julia> LayerNorm(10)
LayerNorm(10)       # 20 parameters

julia> MultiHeadAttention(64 => 1024 => 1024, nheads = 8)
MultiHeadAttention(64 => 1024 => 1024; nheads=8)  # 1_245_184 parameters

This PR:

julia> LayerNorm(10)
LayerNorm(
  identity,
  Scale(10),                            # 20 parameters
  1.0f-5,
  10,
  true,
) 

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 33.56%. Comparing base (e2b3f06) to head (a0b083a).

Files with missing lines	Patch %	Lines
src/layers/macro.jl	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2532      +/-   ##
==========================================
+ Coverage   33.54%   33.56%   +0.01%     
==========================================
  Files          31       31              
  Lines        1881     1871      -10     
==========================================
- Hits          631      628       -3     
+ Misses       1250     1243       -7     

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[CarloLucibello]

Ok now we have a :noexpand option but :expand becomes the default. I remove from the codebase (src+tests) 21 :expand, and added 2 :noexpand (LayerNorm + MultiHeadAttention).

I'm not sure only LayerNorm and MultiHeadAttention need :noexpand, will do more testing.