[misc] fix: sft SFT E2E CI test failure due to megatron engine

[houminz]

What does this PR do?

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Fix the SFT E2E CI test failure caused by the Megatron engine integration (issue introduced in #3600).

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[gemini-code-assist]

Code Review

This pull request effectively resolves a CI test failure within the SFT E2E tests concerning the Megatron engine. The primary fix, located in verl/models/mcore/util.py, correctly introduces a continue statement. This prevents a logical fall-through in the preprocess_packed_seqs_no_padding function when cp_size <= 1, which is a critical correction. The other modifications, such as removing a debug print statement and enhancing comments by replacing FIXMEs with detailed explanations, improve the code's clarity and maintainability. The changes are accurate and well-implemented, making the PR ready for merging.