feat(training): enable MFSDP V2 gradient clipping

[wujingyue]

Summary

• allow clip_grad > 0 for MFSDP V2
• let the dense and EP=2 V2 smoke tests use the normal clipping default (1.0)

Dependencies

• Depends on the MFSDP V2 EP integration in feat(training): support MFSDP V2 expert parallelism #5521.
• Depends on MCore EP support in feat(fsdp): support expert parallelism in MFSDP v2 adapter NVIDIA/Megatron-LM#6450.
• MCore clipping coverage is added separately in fix(optimizer): support MFSDP v2 gradient clipping across dense and expert meshes NVIDIA/Megatron-LM#6489.

Validation

• MCore two-rank dense and EP=2 clipping coverage in fix(optimizer): support MFSDP v2 gradient clipping across dense and expert meshes NVIDIA/Megatron-LM#6489.
• uv run pre-commit run --all-files
• Bridge two-rank smokes were not runnable in this worktree: its committed MCore submodule is intentionally pinned before #6450, and the isolated local environment failed while building fast-hadamard-transform because it does not declare Torch as a build dependency.

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