[megatron] fix: update patch for MLA flashattn forward

[HollowMan6]

What does this PR do?

Now NVIDIA/Megatron-LM@5dcda19 has already been merged into main, so the patch becomes optional when mcore version is greater or equal than 0.16.2

• [MLA] fix: Pad V when Q/V head dims differ for THD NVIDIA/Megatron-LM#3003

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[Copilot]

Pull request overview

This PR updates the Megatron mcore MLA (MultiLatentAttention) forward patch logic so it is only applied for mcore versions older than 0.16.2, aligning with the upstream fix merged into Megatron-LM.

Changes:

• Add an mcore version gate (>= 0.16.2) to skip applying the local MultiLatentAttention.forward patch.
• Refine THD packed-sequence handling by padding/slicing V only when Q/V head dims differ, tracking the original V dim for output slicing.

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[gemini-code-assist]

Code Review

This pull request updates the Megatron-Core patching logic to support versions 0.16.2 and later, while refactoring the patch_forward method to handle cases where query and value head dimensions differ in THD packed sequences. The review feedback identifies a potential AttributeError when accessing configuration attributes and a logic regression where the DSA attention variant might incorrectly trigger post-attention reshaping. Suggestions were provided to use getattr for safer attribute access and to restore the exclusion of the DSA variant in the affected block.