[Bugfix] Scale FlashInfer b12x capacity for data parallelism

[dumko2001]

Purpose

FlashInfer b12x sizes its wrapper from one rank's token capacity, while data
parallel dispatch can gather tokens from every rank. Use the existing
fi_moe_largest_bucket helper so the capacity scales with dp_size and
retains FlashInfer's 8192 minimum.

Fixes #47982

Related work: gh search prs 47982 --repo vllm-project/vllm --match body
returned no PR for this issue. AI assistance was used; I reviewed the changed
code and the available GPU results.

Test Plan

uv tool run ruff check repro/gpu_validate_b12x_dp_sizing.py
uv run --no-project python -m py_compile \
  repro/gpu_validate_b12x_dp_sizing.py
python repro/gpu_validate_b12x_dp_sizing.py

The probe was run against the stock wheel and then after overlaying:

vllm/model_executor/layers/fused_moe/experts/flashinfer_b12x_moe.py

Added CPU unit coverage for DP-scaled bucket sizing and the optional tuning
floor used by the helper. The follow-up commit passes min_num_tokens=0 for
the B12x wrapper so the helper does not reintroduce the fixed floor.

The helper check ran on the earlier H100 pod and on a two-RTX-5090 SM120
pod. The latter had two visible GPUs. The b12x launch was also attempted
directly on SM120.

Test Result

The helper produced the DP-scaled capacity on both pods:

EXPECTED_DP_CAPACITY: 8192
HELPER_CAPACITY: 8192
PASS: DP capacity helper scales by dp_size

The two-RTX-5090 run also reported:

CUDA_DEVICES: 2
PASS: two CUDA devices available

Secure-cloud follow-up on an RTX 5090 with driver CUDA 13.0 reached and
executed the real SM120 b12x kernel. The FlashInfer version guard reported
CUDA 12.8 inside the process, so the direct probe bypassed that guard only to
exercise the already-installed SM120 kernel.

Unpatched:

B12X_VARIANT: UNPATCHED
CONFIGURED_CAPACITY: 4096
REQUIRED_DP_CAPACITY: 8192
EFFECTIVE_WRAPPER_CAPACITY: 4096
EXPECTED: unpatched wrapper capacity is too small for DP=2
PASS: b12x kernel output torch.Size([1, 128]) torch.bfloat16 75776.0

Patched:

B12X_VARIANT: PATCHED
CONFIGURED_CAPACITY: 4096
REQUIRED_DP_CAPACITY: 8192
EFFECTIVE_WRAPPER_CAPACITY: 8192
PASS: patched wrapper capacity covers DP=2
PASS: b12x kernel output torch.Size([1, 128]) torch.bfloat16 65536.0

The helper probe continued to report HELPER_CAPACITY: 8192. The secure
cloud pod had one GPU, so a distributed DP=2 launch was not attempted.

---

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