feat: support DeepSeek V4 THD packing

[samaritan1998]

Summary

• Add true THD packing for DeepSeek-V4 with sample-local cu_seqlens boundaries.
• Keep CP all-gather, DSV4 window/compressed attention, compressor overlap, RoPE, and TileLang indexer paths boundary-safe.
• Propagate per-sample padded lengths through response extraction, CP loss masks, log-probs, entropy, and DeepSeek-V4 assistant-only loss masking.
• Add focused packing, CP-boundary, compressor, indexer workspace, padding-alignment, and loss-mask tests.

Validation

• LTP job: zmm-dsv4-thd-tp2p8c4-fp32-r13-0722-9e5d50
• Cluster: book_tjzj_bzz2_hpn1
• Topology: TP2 / PP8 / CP4 / EP4, 64 GPUs, qkv_format=thd, micro_batch_size=2
• LTP result: Succeeded
• Runtime evidence: 7 passed; official torch_dist checkpoint loaded; Final collected 2 samples; step 0 train/loss=0.1832201839, train/grad_norm=1.8685250699; actor train completed normally.
• Static validation: Ruff, Python compileall, and git diff --check.

[gemini-code-assist]

Caution

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