fix checkpointing conversion

[dimapihtar]

What does this PR do ?

Fix for #3240.

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[dimapihtar]

/ok to test fa2e133

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23792215872

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/23793689232