[Kernel] Resurrect rotted gather-cache OOB test + defensive block-table bound-checks

[waynehacking8]

Purpose

Restore a dead regression test and add two defensive bound-checks in the CP gather cache kernels (csrc/libtorch_stable/cache_kernels.cu).

1. Resurrect a dead regression test (the substantive fix). tests/kernels/test_cache_kernels.py::test_gather_cache_oob (the [Bug]: Potential Out-of-bounds in cache_kernels.cu #27909 sanitizer-mode regression test) still called the op without the since-added token_to_seq argument and fails with a binding error on current main (Expected a value of type 'Tensor' for argument 'token_to_seq' but instead found type 'int'). The only sanitizer-mode coverage for this kernel family has been dead since the signature changed; updated to the current signature.

2. Defensive bound-checks for two malformed-index paths ([Bug]: Out of bounds in gather_and_maybe_dequant_cache #45380, [Bug]: Out of bounds in cp_gather_and_upconvert_fp8_kv_cache #45377). Both kernels index block_table[...] with a computed offset that isn't range-checked:

   • gather_and_maybe_dequant_cache: block_table_id can exceed the table width if seq_starts pushes a token past the block table's coverage ([Bug]: Out of bounds in gather_and_maybe_dequant_cache #45380). The kernel now receives block_table.size(1) and continues past out-of-range indices.
   • cp_gather_and_upconvert_fp8_kv_cache: the ownership binary search floors req_id at 0, so a nonzero workspace_starts[0] would drive token_offset negative and underflow the read ([Bug]: Out of bounds in cp_gather_and_upconvert_fp8_kv_cache #45377). Early-return on a negative offset (warp-uniform).

   Reachability (honest): I could not establish that either input occurs in production. For [Bug]: Out of bounds in cp_gather_and_upconvert_fp8_kv_cache #45377 I traced the callers — flashmla_sparse.py builds workspace_starts as zeros() then cumsum() into [1:], with the per-chunk adjustment zeroing each chunk slice's first entry, so workspace_starts[0] == 0 always and token_offset >= 0 by construction. For [Bug]: Out of bounds in gather_and_maybe_dequant_cache #45380 the chunked-context caller (mla_attention.py) sizes the block table to the full sequence, so a well-formed config keeps block_table_id in range. The sanitizer evidence below is from feeding the issues' constructed inputs directly. So these are defensive guards against malformed indices into block_table, not fixes for demonstrated production failures — happy to drop either if you'd prefer not to guard a currently-unreachable path.

Test Plan / Result

• tests/kernels/test_cache_kernels.py — 2 passed (previously 1 failed on main from the signature rot). Against the prebuilt binary these exercise the op bindings; CI's source build + sanitizer covers the guards.
• Standalone compute-sanitizer (--tool memcheck, sm_120, CUDA 13.0) on a replica of the kernels' indexing logic (payload simplified to plain copies; the indexing under test preserved verbatim), fed the issues' constructed trigger inputs:

build	constructed trigger inputs	valid inputs
pre-fix logic	4161 invalid global reads	0 errors
with guards	0 errors	0 errors

The guards are pure bound checks, so behavior on well-formed input is unchanged.

Duplicate check

No open PR addresses either issue: searched 45380, 45377, gather_and_maybe_dequant, cp_gather_and_upconvert; both issues have zero comments; reporter has no authored PRs; file history on main shows no fix since the libtorch_stable migration. Related but distinct: #45384 fixes an int32 overflow in concat_mla_q in the same file (different function; no textual overlap).

AI assistance disclosure

Prepared with AI assistance (Claude); reviewed, compiled, and sanitizer-validated by the submitter.

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[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @waynehacking8.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[waynehacking8]

Rebased on latest main to resolve the conflict with #48642:

• Kept the negative-offset guard in cp_gather_and_upconvert_fp8_kv_cache, now applied to the final token_offset after the new seq_starts adjustment (guarding the final value is what protects the block_table read from underflow).
• Adapted the [Bug]: Out of bounds in cp_gather_and_upconvert_fp8_kv_cache #45377 repro test to the new op signature (seq_lens removed, total_tokens now derived from dst.size(0)).

cc @MatthewBonanni since this touches the same kernel as #48642 — PTAL.

Could a maintainer add the ready label to trigger CI? Thanks!

[MatthewBonanni]

This resolves #45377 but I don't think this PR is necessary. It's a defense against something that shouldn't happen in production. Also, as is, this just kind of kicks the can down the road because it lets the invalid rows stay uninitialized