Use float intrinsics for deform_conv2d backward, fix into_data for padded tensors

[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.

Changes

Uses float intrinsics instead of CAS where available
Fixes into_data for padded tensors by slicing to the expected length before creating the TensorData.

Testing

All tests pass except for remainder on wgpu/spirv, which is still broken due to a driver bug.

[codecov]

Codecov Report

Attention: Patch coverage is 74.25743% with 26 lines in your changes missing coverage. Please review.

Project coverage is 82.90%. Comparing base (51b742f) to head (9bbd495).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
...urn-jit/src/kernel/conv/deform_conv_transpose2d.rs	74.19%	24 Missing ⚠️
...tes/burn-jit/src/kernel/conv/conv2d/gemm/launch.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2681      +/-   ##
==========================================
- Coverage   82.91%   82.90%   -0.02%     
==========================================
  Files         843      843              
  Lines      107431   107461      +30     
==========================================
+ Hits        89081    89093      +12     
- Misses      18350    18368      +18     

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[laggui]

Might have missed a comment from debugging