[fix]: fix seq_len pad len, and adapt to new mtp_loss api (for megatron dev brance)

[zpltys]

1. when both enable CP and TP, seq_len should pad to (2 * cp) * tp
2. adapt to new mtp_loss api (for megatron dev brance)

What does this PR do?

Bugfix:
1. when both enable CP and TP, seq_len should pad to (2 * cp) * tp
2. adapt to new mtp_loss api (for megatron dev brance)

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[gemini-code-assist]

Code Review

This pull request updates the Multi-Token Prediction (MTP) loss calculation to support the new Megatron API while maintaining a legacy fallback. It also adjusts the sequence length alignment logic in utility functions to ensure compatibility with zigzag Context Parallel (CP) and Tensor Parallel (TP) configurations by calculating align_size as tp_size * cp_size * 2. I have no feedback to provide.