[BREAKING][megatron] chore: set ETP default to null

[HollowMan6]

What does this PR do?

So that by default, it will follow the TP size.

Related to #4063 (comment)

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[gemini-code-assist]

Code Review

This pull request changes the default value for expert_tensor_parallel_size from 1 to null across Megatron configurations. This is intended to make it automatically follow the tensor_model_parallel_size (TP size). The change is consistent across all modified configuration files. However, this introduces a breaking change for users who have custom configurations with tensor_model_parallel_size > 1, as the behavior of expert_tensor_parallel_size will change. I've added a comment to recommend that this pull request be marked as a breaking change to ensure users are aware of the new default behavior.