feat(qwen35): shard GDR state per TP rank + batched eager/graph decode under TP (+27B benchmark validation) - #946

Phase 2b ported onto pegainfer-project#870: recurrent/conv state, GDR scratch, and the
linear-attention weight surface are allocated and addressed per rank
instead of replicated, which is what makes 27B TP2 fit on 2x48 GB cards.

- weight_loader: additive stitch/shard loaders (2D row stitch for the
  fused qkv [q|k|v] segments, 1D element stitch for conv1d channels,
  bf16/f32 1D shards for dt_bias/A_log)
- config: local_linear_* accessors mirroring the full-attn TP style;
  linear head divisibility fails closed in TensorParallelConfig
- weights: per-rank stitched shards for in_proj_qkv/conv1d (head-local
  slices per segment), row shards for z/b/a, col shard for out_proj,
  dt_bias/A_log sliced, norm_weight kept replicated (head-shared);
  loader reserve uses TP-aware estimates
- recurrent_state/decode_buffers/prefill_buffers: state and GDR scratch
  at local value-head/qkv sizes; capacity math derives from local
  allocation_bytes
- batch_decode/prefill: local head counts into the GDR decode/conv/
  Triton-AOT prefill chains, gated RMSNorm at local v heads, all-reduce
  after linear out_proj; batch_decode_full_attention_via_prefill is now
  TP-local so eager decode routes 27B TP2 group-6 full attention through
  prefill (was FlashInfer Unsupported group_size: 6)
- tp_executor: worker capacity math and per-request state use the
  rank-local sizes; decode rows still run as a per-request bs=1 loop
  (batched in a follow-up)

Recurrent/conv state is never all-reduced.

Signed-off-by: Ziyang Zhang <hafuhafu@qq.com>

P2c from docs/models/qwen35/tp-design.md. TP decode captures/replays
per-bucket CUDA Graphs when --cuda-graph is set AND the TP-local decode
GQA group has a compiled kernel (4B/9B TP2); 27B TP2 (group 6) keeps the
batched eager path byte-for-byte under the gate.

- Gate: drop the fail-closed TP+graph rejections in config.rs/lib.rs/
  tp_executor.rs; log once when graph was requested but the group gate
  keeps decode eager.
- State: scheduler owns dense decode slots (slot_idx on TP decode rows,
  slot_for_new_request at promote, compaction_after_retire on retire);
  workers hold a fixed-address BatchDecodeGraphState plus slot_map, D2D
  copy prefill state into the slot on the first decode row, and apply
  DropRequest compactions via move_slot_within with occupancy assertions
  (poison on mismatch).
- Capture/replay: startup pre-capture sweep ported from qwen3 (Warmup
  via new warmup_tp_collective, Capture/Launch per bucket, Finalize)
  behind a 600 s abort watchdog; batch_decode_graph gains DecodeGraphUse
  (Serve/CaptureOnly/Replay); TP serving is replay-only; graphs drop
  before the NCCL comm.
- Convenience executor API (model-local tests) keeps a slot tracker
  mirroring Qwen35Executor; scheduler flows pass explicit slots.

Validation (2x RTX 4090, venv NCCL): lib 105/105; 9B TP2 HF gates eager
+ graph (sequential replay, bucket-straddling, post-compaction) pass
within existing tolerances; 9B TP2 scheduler e2e eager+graph pass;
serving_tp2 now launches with graph on; 27B TP2 HF+e2e pass unchanged
(group-6 gate keeps eager, graph variant self-skips). Serving benchmark
9B TP2, 16 concurrent x 256 out: 767.15 tok/s graph vs 705.86 eager
(+8.7% steady output, TPOT 20.04 vs 21.99 ms).

Signed-off-by: Ziyang Zhang <hafuhafu@qq.com>