PyTorch fixes AOTInductor crash on incompatible tile fusion

PyTorch merged a fix for a bug in `_split_iteration_ranges` within `torch/_inductor/codegen/simd.py` that caused AOTInductor to crash during compilation when attempting to fuse a pointwise epilogue with incompatible tiling groups. The function is responsible for determining whether a pointwise kernel can be fused into an existing template (e.g., matmul) tile by splitting iteration ranges. Under certain conditions—specifically when the epilogue's iteration domain is a strict sub-multiple of the template's tiling—the function would hit a bare `assert all(... == 1 for s in remaining)` at the end. This assertion failed with an `AssertionError` instead of raising `CantSplit`, the exception used elsewhere in the same function to signal incompatible tiling. Crucially, both callers driving epilogue fusion (`SIMDKernel.is_compatible` and `Scheduler.speedup_by_fusion`) wrap the `_split_iteration_ranges` call in `try/except CantSplit` blocks to gracefully fall back to unfused codegen. Because the final assert threw `AssertionError`, it escaped those handlers and caused AOTInductor to hard-fail with `InductorError: AssertionError: failed to set ranges ...`.

The fix converts that final assert to `raise CantSplit(remaining, lengths)`, matching the exception contract of the rest of the function. The success path is byte-for-byte unchanged—models that compiled correctly today remain unaffected. Models that previously crashed now compile successfully, with the incompatible epilogue left unfused as a separate kernel instead of aborting the compile entirely. The fix includes a new regression test, `test_leftover_extent_raises_cant_split`, which constructs groups `[2, 2]` and lengths `[[2], []]`—a scenario where all sizes divide cleanly but group 1 is left with extent 2—and asserts that `CantSplit` is raised rather than `AssertionError`. This patch improves AOTInductor reliability by ensuring that tiling-incompatible fusions are skipped gracefully rather than crashing the compiler.