[megatron, training_utils] fix: encoder pp is removed in mcore >= 0.14

[HollowMan6]

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Implementation refers to NVIDIA/Megatron-LM@b600e38#diff-ff40c5dfa6c8106a478517375d98bc4e548ff71bcc3e5b25a4c1cc540f31ed3a

Use hasattr(parallel_state, "is_inside_encoder") for backward compatibility.

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[gemini-code-assist]

Code Review

This pull request adds backward compatibility for mcore>=0.14 where encoder pipeline parallelism support was removed. The change correctly uses hasattr to check for the existence of parallel_state.is_inside_encoder. However, the check is incomplete as it doesn't guard against a missing parallel_state.get_pipeline_model_parallel_decoder_start function, which could lead to a crash. I've added a suggestion to make the condition more robust.