[Fix] gptoss yarn parameter

[yiakwy-xpu-ml-framework-team]

What does this PR do ?

Fix gptoss GptOSS yarn default parameters when passing them to mcore where mcore does not accept None as valid vlaues.

Changelog

• Fix GptOss Yarn default parameters : mcore cannot read none values

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Additional Information

• Related to [Feature] add gptoss continue train bf16-fp8 (sft) example [part1 - mcore] NVIDIA/Megatron-LM#2383

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[yiakwy-xpu-ml-framework-team]

Doctor sergiopperez Could you have a look at this ?

This is essentail for support of GptOSS traininig D.