fix(mhc): update CUDA graph module API

[FDecaYed]

fix(mhc): update CUDA graph module API

Summary

Update HyperConnectionTransformerLayer._get_submodules_under_cudagraphs() to use the
current CUDA graph configuration API:

• cuda_graph_modules instead of the deprecated cuda_graph_scope
• CudaGraphModule instead of the removed CudaGraphScope

This is a compatibility-only follow-up. It does not add new mHC or DSv4 behavior.

Stack

• Base: [Split 1/N of #3430] feat(mHC): basic pytorch implementation of manifold hyper connection #4531 (pull-request/4531)
• Base commit: f1df30c1db8c2ece4df1e0465b89f67f12337e7b
• Related integration umbrella: [Tracking] DeepSeek-v4 HybridModel merge plan #5795

This PR should merge after #4531. Before #4531 merges, retarget this PR to main
and refresh it against the merged result so GitHub does not automatically close
the dependent PR.

Credit

The mHC implementation being updated was authored by @Connor-XY in
#4531. This follow-up only adapts that implementation to the
current CUDA graph module API needed by the #5795 integration.

Validation

• uv run ruff check megatron/core/transformer/transformer_layer.py
• uv run python -m py_compile megatron/core/transformer/transformer_layer.py
• git diff --check
• Targeted pytest collection attempted:
  tests/unit_tests/transformer/test_transformer_layer.py::TestMHCWithCudaGraph::test_submodules_under_cudagraphs_includes_hyper_connection
  (blocked locally because the available environment does not include torch)

Checklist

• One compatibility fix in one production file
• Original author and PR credited
• [Tracking] DeepSeek-v4 HybridModel merge plan #5795 referenced for integration context
• Intended to be opened as a draft

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[FDecaYed]

/ok to test 369b79e

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[FDecaYed]

/ok to test 672202b