Fix split_state_dict function for MoE models

[eternally-z]

What does this PR do ?

main PR: #3676

This PR fixes an issue in _split_state_dict, which splits the state dict into sub-state dicts for chained optimizers.

The previous offset-based implementation failed in MoE scenarios. Since MoE parts often have a separate optimizer, the offset-based splitting logic caused chunk mis-assignment. As a result, the MoE optimizer was unable to obtain the correct state_dicts.

@AniZpZ

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[kunlunl]

/ok to test c65fd5c

[kunlunl]

/ok to test e867137

[kunlunl]

@eternally-z The UT failed. Please take a look.

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/22883926348