[mcore] Dynamic CP group for training with sequence packing

**Is your feature request related to a problem? Please describe.**
In long-context GRPO, the sequence length distribution in a batch can be very skewed, and always using a large CP to tolerate the longest sequence is inefficient for most short sequences.

This issue requests for dynamic CP group feature that can dynamically arrange DP and CP group size per batch to reduce exposed CP communication.

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