[Fix] Avoid rebinding chunk loop variables for int64 anchoring

[zhiyuan1i]

Summary

Triton shipped with PyTorch 2.7 rejects re-assigning a loop induction variable with a different type:

AssertionError: Loop-carried variable i_t has initial type int32 but is re-assigned to int64 in loop!

Since #1173, eleven chunk kernels cast the loop variable in place (i_t = i_t.to(tl.int64)), so ops using them fail to compile under the H100 PyTorch 2.7 CI env (seen on #1184 and #1111, e.g. tests/ops/test_precond_gated_delta.py::test_chunk). Use a fresh i_t_int64 variable instead, matching the convention already used in chunk_delta_h.py. Semantics unchanged.

Test plan

• Unit tests added/modified: none (no behavior change).
• Dependent tests run: python scripts/find_dependent_tests.py over the 5 changed files (30 test files across ops/layers/context_parallel).
  • torch 2.7.1 + triton 3.3.1 (same versions as the failing CI env): pre-fix reproduces the exact CI failure (test_precond_gated_delta.py::test_chunk, same failing case, 8 passed / 1 failed); post-fix the full file passes (38/38).
  • torch 2.10 (newer Triton): test_ttt.py, test_linear_attn.py, test_retention.py pass.
  • The full 30-file dependent suite on torch 2.7.1 is running; this PR's own PyTorch 2.7 CI job covers the same scope.
• Varlen / CP / model tests: the dependent list includes tests/context_parallel and varlen cases; all exercised by CI.

Benchmark / NCU (kernel changes only)

Neutral — pure variable rename, no performance-related change.

Breaking changes

None.

Checklist

• I have read CONTRIBUTING.md and follow its conventions (code style, docstrings, commit prefixes).
• I have read AGENTS.md and, where my change matches its scope, the relevant skill under .agents/skills.
• Dependent tests pass locally or in CI, and new behavior is covered by tests where applicable (tick as N/A for changes with no testable code, e.g. docs-only).
• Kernel changes include same-hardware before/after benchmark numbers, dense + varlen where applicable (tick as N/A when no kernel code changed).
• This PR is minor/cosmetic-only (typo, formatting, style-only tweaks) — tick only if it is, and justify below.