save train state for checkpoints

[dimapihtar]

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What does this PR do?

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• I have added relevant unit tests
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[maanug-nv]

/ok to test 576d8df

[dimapihtar]

/ok to test b813a9c

[maanug-nv]

/ok to test 76e6a93

[dimapihtar]

/ok to test 07f2e2b

[nemo-automation-bot]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/37565288951