[Relax][PyTorch] Support aten.scatter.src in ExportedProgram importer

[V-aerus]

This PR adds PyTorch ExportedProgram frontend support for aten.scatter.src by mapping it to the existing scatter conversion path.

The importer already supports scatter.value, and BaseFXGraphImporter already provides _scatter for tensor-source scatter. This patch registers scatter.src in the ExportedProgram convert map and adds a regression test.

Test:
python -m pytest tests/python/relax/test_frontend_from_exported_program.py -k "scatter" -q

[gemini-code-assist]

Code Review

This pull request adds support for the scatter.src operator in the TVM Relax PyTorch exported program translator by mapping it to the _scatter translator method. It also includes a unit test test_scatter_src to verify the correct translation of the operator. There are no review comments to address.

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