[Bugfix] Guard fabric handle APIs for CUDA < 12.4 compatibility

[aimbit-ni]

Summary

• CU_DEVICE_ATTRIBUTE_HANDLE_TYPE_FABRIC_SUPPORTED and CU_MEM_HANDLE_TYPE_FABRIC were introduced in CUDA 12.4
• Without a version guard, cumem_allocator.cpp fails to compile on CUDA 12.0–12.3
• Wraps the fabric detection block in #if CUDA_VERSION >= 12040 so older toolchains skip fabric support gracefully
• fab_flag remains initialized to 0, so the existing fallback logic at line 130 is unaffected

Introduced in #33540.

Test plan

• Compiles cleanly on CUDA 12.0 (V12.0.140) — fabric block skipped
• Compiles cleanly on CUDA 12.8 (V12.8.61) — fabric detection still active
• pre-commit run --files csrc/cumem_allocator.cpp passes (clang-format, typos)

Signed-off-by: Nikolaos Ioannidis ni@aimbit.de

[github-actions]

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[gemini-code-assist]

Code Review

This pull request correctly addresses a build failure on CUDA versions older than 12.4 by adding a preprocessor guard (#if CUDA_VERSION >= 12040) around code that uses APIs specific to CUDA 12.4 and later. The change is well-contained, uses the standard version macro, and correctly maintains the existing logic for older toolchains. The fix is appropriate and I see no issues with the implementation.

[mergify]

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

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

[github-actions]

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