[Bugfix] Prevent NaN poisoning in xpu_mla_sparse for fully-masked index chunks

[nickus]

FIX #48364

Purpose

_bf16_mla_sparse_kernel (the XPU sparse-MLA kernel behind XPU_MLA_SPARSE, DeepSeek-V4 XPU prefill, and the fp8 decode wrapper) NaN-poisons its output whenever the first BLOCK_N (=16) topk index entries of a row are all masked, even though valid keys follow later:

• the running max starts at -inf, and a fully-masked chunk sets every logit to -inf;
• re_scale = exp2(-inf - -inf) = NaN then poisons acc and e_sum permanently (nan * 0 = nan), so later valid chunks cannot recover the row.

A row with no valid key at all likewise returns NaN (0/0), where the in-tree reference (reference_mla_sparse_prefill) explicitly defines zeros.

The fix replaces -inf with a finite sentinel (-1.0e30) for the running-max init and the masked logits — the same approach the newer sparse-MLA kernel in #47629 uses. Two lines, no perf change:

• chunks mixing valid and masked keys are unaffected: exp2(-1e30 - m) is exactly 0.0 in fp32;
• leading fully-masked chunks are rescaled away exactly once a valid chunk arrives;
• rows with no valid key produce zeros, matching the reference semantics.

The existing test never catches this because it writes valid indices at the front of each row and pads with -1 at the back — trailing padding is benign. The added regression test covers leading + trailing masked chunks and a fully-masked row.

Test Plan

• Minimal repro in [Bug]: xpu_mla_sparse NaN-poisons attention output when a row's leading topk index chunk is fully masked #48364 (runs on any GPU, or CPU via TRITON_INTERPRET=1 — it is a pure numerics bug).
• New test_bf16_triton_sparse_mla_masked_chunks: verified bidirectionally on the kernel via the Triton interpreter — it FAILS (NaN) on the unfixed kernel and passes on the fixed one, with out/max_logits/lse matching reference_mla_sparse_prefill within the file's existing tolerances.
• test_bf16_triton_sparse_mla (existing) is unaffected: for any row containing at least one valid key in its first chunk, the sentinel path computes bit-identical p/re_scale values.

Test Result

[broken] out finite: False            # regression test catches the bug
[fixed]  out finite: True
[fixed]  out[:2]  allclose vs reference: True  (max |d| = 0.0075, bf16 store rounding)
[fixed]  max_logits allclose: True  (max |d| = 2e-6)
[fixed]  lse        allclose: True  (max |d| < 1e-6)
[fixed]  fully-masked row == zeros: True

🤖 Generated with Claude Code

[claude]

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[jikunshang]

cc @wuxun-zhang PTAL

[wuxun-zhang]

@majian4work Could you please try with DeepSeek model to see if any accuracy regression?

[majian4work]

Thanks @nickus , good catch.
The fix LGTM, but the GSM8k drops a bit (0.946 -> 0.94); maybe it's noise.

[wuxun-zhang]

LGTM

[nickus]

Guys, it seems i cannot merge. I do not have write permission to the repo.
Could somebody assist me on this?