Add converter for aten::_grouped_mm.default

[Sid-V5]

Implemented the converter for aten::_grouped_mm.default to address #2795.

Changes

• Added aten_grouped_mm function in onnxscript/function_libs/torch_lib/ops/core.py

Implementation Details

The aten::_grouped_mm operator performs grouped matrix multiplication. This implementation handles the batch/dense mode (when offs is None), where groups are implicit in the batch dimension:

• self: (G, M, K), mat2: (G, K, N) → result: (G, M, N)
• Uses op.MatMul for the core computation
• Supports optional bias addition via op.Add
• Supports optional out_dtype casting via op.Cast

The offset-based mode (when offs is provided) raises NotImplementedError, as it requires segment-level matrix multiplications that are not directly expressible with standard ONNX operators.

Testing

The function follows the same patterns as other converters in core.py (e.g., aten_bmm, aten_mm) and uses the @torch_op decorator for automatic registration.

Fixes #2795

[Sid-V5]

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