[tandede]

Fixes #20853.

PyTorch defines add, sub, and rsub as applying alpha to the second operand before performing the binary operation. The exported program importer previously discarded this keyword argument, silently producing incorrect Relax IR whenever alpha != 1.

This change:

• scales the appropriate operand before lowering add, sub, and rsub;
• handles tensor and scalar operands;
• adds support for sub_.Tensor, matching the existing add_.Tensor handling; and
• adds structural regression coverage for functional and in-place forms.

Testing:

• pytest tests/python/relax/test_frontend_from_exported_program.py -k "binary1 or binary_scalar or add_sub_alpha"
• pre-commit run --files python/tvm/relax/frontend/torch/base_fx_graph_translator.py python/tvm/relax/frontend/torch/exported_program_translator.py tests/python/relax/test_frontend_from_exported_program.py