[Test] Fix one-sided MNNVL alltoall test workspace under-reservation

[zyongye]

Summary

tests/distributed/test_mnnvl_alltoall.py::test_one_sided_dispatch_combine
initializes the FlashInfer one-sided MoeAlltoAll workspace without
declaring the fp8 block-scale payload it later dispatches. The worker
dispatches four payloads:

• a1q — nvfp4 hidden states, (tokens, hidden // 2) → hidden // 2 B/token
• a1q_scale — fp8 block scales, (tokens, hidden // 16) → hidden // 16 B/token
• topk_ids, topk_weights — top_k * 4 B/token each

but manager.initialize(...) is called with dispatch_scale_bytes_per_token
left at its default of 0. So the reserved per-rank dispatch region is short
by hidden // 16 bytes/token. That offset shortfall pushes the combine
payload region past the end of the per-rank workspace, and FlashInfer's
combine guard (trtllm_moe_alltoall.cu) trips:

Check failed: (combinePayloadOffset >= 0 && combinePayloadOffset + payloadBytes <= sizePerRank)
workspace insufficient for combine payload region

The per-rank workspace is sized as
aux + ep_size·max_tokens·dispatch_payload_per_token + ep_size·max_tokens·combine_payload_per_token,
so under-declaring the dispatch payload directly under-reserves the buffer.

Fix

Pass dispatch_scale_bytes_per_token=hidden_size // 16 to initialize() so
the reserved dispatch region matches the payloads the test actually
dispatches. One-line, test-only change.

Testing

On a 2-GPU MNNVL host, run with CAP_SYS_PTRACE (MNNVL needs it for IPC; the
tests @requires_ptrace-skip otherwise):

pytest tests/distributed/test_mnnvl_alltoall.py -v

• Before: 5 passed, 1 failed (test_one_sided_dispatch_combine →
  "workspace insufficient for combine payload region").
• After: 6 passed (all manager-lifecycle, workspace-grow, args/two-sided/
  one-sided dispatch-combine tests).

AI assistance

This change was developed with AI assistance (Claude Code). The diff has been
reviewed and the test result above was run locally.