fix(mtp): support contiguous context-parallel THD rolls

[FDecaYed]

What

Extend MTP packed-sequence rolling so contiguous context-parallel THD shards roll across rank boundaries without crossing packed-sequence boundaries.

• retain the existing zigzag path as the default;
• dispatch through the duck-typed cp_partition_mode contract used by the dynamic-CP stack;
• exchange only the boundary token needed by adjacent contiguous ranks;
• allow callers to choose the boundary fill value, including True for padding masks;
• cover values and boolean padding masks on two contiguous CP ranks.

Stack

This PR is stacked on #4495, which fixes padded cumulative-sequence boundaries for MTP THD rolling.

The future dynamic-CP integration supplies PackedSeqParams.cp_partition_mode; this focused PR deliberately does not pull that broader data/attention stack into MTP. Its test exercises the same duck-typed contract directly.

Provenance

• [Dev] fix(mtp): use padded cu_seqlens in MTP roll for THD with CP #4494 and fix(mtp): use padded cu_seqlens in MTP roll for THD with CP #4495 provide the original padded-cu-seqlens fix and external prerequisite.
• The boundary-fill contract is adapted from the MTP portion of [dev] [DeepSeek-v4] Packed Sequence (THD) support for DSv4 Hybrid Attention #5011.
• The contiguous rolling behavior refactors commit 47ee3b2 from the frozen feature set collected in [Tracking] DeepSeek-v4 HybridModel merge plan #5795.
• Original implementation by @hxbai.
• Reconstructed against the fix(mtp): use padded cu_seqlens in MTP roll for THD with CP #4495 merge mirror; feature cutoff: 2026-07-21.
• This is not a mechanical replay or rebase, and adds no GPTModel integration.

Testing

• isort --check-only, ruff, Black --check, Python compilation, and git diff --check passed for both changed files.
• Two-rank CUDA execution is delegated to PR CI because this local macOS host has no CUDA devices.

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[FDecaYed]

/ok to test f57f2df

[FDecaYed]

Closing this draft until Round 1. Its logical successor is P9 (contiguous-CP packed MTP roll), rebuilt on main after external prerequisite #4495 merges. Tracking PR: #5795.