[torchlib] Fix aten.embedding_bag.padding_idx to support None values

[Copilot]

The aten.embedding_bag.padding_idx function was failing when padding_idx is None, which is a valid value in PyTorch indicating that no padding index should be ignored. This was causing DLRM model exports to fail with the error:

padding_idx must not be None. This is likely a dispatcher error

Changes Made

1. Updated function signature: Changed padding_idx: int = -1 to padding_idx: Optional[int] = None
2. Removed problematic assertion: Eliminated the assertion that was throwing the error when padding_idx is None
3. Added conditional logic:
   • When padding_idx is None: delegate to _aten_embedding_bag_onnx() (regular embedding_bag without padding filtering)
   • When padding_idx is not None: use existing _aten_embedding_bag_1d_padding_idx_onnx() implementation

Behavior

# This now works correctly (previously threw assertion error)
result = aten_embedding_bag_padding_idx(
    weight, indices, offsets, 
    padding_idx=None  # No padding index to ignore
)

# This continues to work as before
result = aten_embedding_bag_padding_idx(
    weight, indices, offsets, 
    padding_idx=2  # Filter out index 2
)

The fix maintains full backward compatibility while correctly handling the None case as expected by PyTorch's behavior.

Fixes #2219.

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[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.36%. Comparing base (b76e1b3) to head (a69a674).
⚠️ Report is 243 commits behind head on main.

Files with missing lines	Patch %	Lines
onnxscript/function_libs/torch_lib/ops/core.py	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2384      +/-   ##
==========================================
- Coverage   70.37%   70.36%   -0.01%     
==========================================
  Files         199      199              
  Lines       25206    25207       +1     
  Branches     2685     2686       +1     
==========================================
- Hits        17739    17738       -1     
- Misses       6537     6538       +1     
- Partials      930      931       +1     

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