[cherry-pick][release/2.13] NCCL ldconfig symlink fix + atomic symlink

[apbose]

Cherry-pick of the last two commits from abose/release_2.12_mdtrt_ld_library_fix onto release/2.13.

Both applied cleanly (release/2.13's _nccl_utils.py was at the same base as release/2.12); the resulting _nccl_utils.py is byte-identical to the source branch's final version.

Commits:

1. c4fce46b3 setup_nccl_for_torch_tensorrt change — replaces the old LD_LIBRARY_PATH + ctypes preload approach with creating a libnccl.so symlink in a system libdir that ld.so already searches (Debian/Ubuntu multiarch, RHEL lib64, or /usr/lib fallback via new helper _sys_libdir_on_ldso_path()), then ldconfig. Removes check_nccl_library_path() and its two tests, and trims the warning in check_nccl_engine_requirements().
2. 55b3ea7d8 atomic symlink — makes the symlink swap race-safe for multi-process/distributed runs: symlink to a unique per-pid temp name then os.replace onto the target, with temp cleanup and "another rank already produced the correct symlink" handling.

Files: py/torch_tensorrt/distributed/_nccl_utils.py, tests/py/dynamo/distributed/test_native_nccl.py

🤖 Generated with Claude Code

[apbose]

This was incorrect. Closing in favor of #4408