Add Hard sigmoid activation function

[wingertge]

Pull Request Template

Checklist

• Confirmed that run-checks all script has been executed.
• Made sure the book is up to date with changes in this PR.

Related Issues/PRs

Required to load the DbNet and PaddlePaddle OCR ONNX models used in RapidOCR.

Changes

1. Add (configurable) hard sigmoid activation function
2. Add ONNX import support for the HardSigmoid node

Testing

New unit tests and end-to-end ONNX tests.

[antimora]

LGTM from my end!

One part that I am not sure about is if we need hard_sigmoid_backward function as well?

@nathanielsimard can you please confirm?

[codecov]

Codecov Report

Attention: Patch coverage is 98.01325% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.27%. Comparing base (f7639bd) to head (a9e480f).
Report is 7 commits behind head on main.

Files	Patch %	Lines
crates/burn-import/src/onnx/op_configuration.rs	80.00%	2 Missing ⚠️
crates/burn-core/src/nn/hard_sigmoid.rs	97.36%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2112      +/-   ##
==========================================
+ Coverage   86.23%   86.27%   +0.04%     
==========================================
  Files         688      690       +2     
  Lines       88465    88634     +169     
==========================================
+ Hits        76285    76473     +188     
+ Misses      12180    12161      -19     

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[wingertge]

One part that I am not sure about is if we need hard_sigmoid_backward function as well?

The only other activation function with a scalar parameter is leaky_relu which also doesn't have a backwards function so I couldn't figure out how to implement it. If there is a way I'd be glad to add it.

[laggui]

Thanks for adding the implementation all the way through ONNX import!

For the backward, I think it's fine for now. We can let autodiff handle it since it's implemented as a combination of other ops.