TL/NCCL: add ncclAlltoAll

[yaeliyac]

What

As described in NCCL 2.28.3 release notes, NCCL added a new alltoall API.
documentation

This PR updates this (while checking valid NCCL version)

[greptile-apps]

Greptile Summary

Updates the NCCL transport layer to use the native ncclAlltoAll API introduced in NCCL 2.28.0, while maintaining backward compatibility with older versions.

• Uses the new ncclAlltoAll API when NCCL 2.28.0+ is available, which replaces the previous manual implementation using ncclGroupStart/ncclSend/ncclRecv/ncclGroupEnd
• Adds proper version guard with NCCL_VERSION_CODE >= NCCL_VERSION(2,28,0) to ensure compatibility
• Removes unused peer variable declaration from the function scope and moves it inline in the fallback loop (C99 style declaration)
• The native API should provide better performance compared to the manual send/recv implementation

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The change is well-contained, properly version-guarded, and maintains full backward compatibility. The implementation correctly uses the new NCCL API with appropriate parameters, and the fallback code remains unchanged for older NCCL versions.
• No files require special attention

Important Files Changed

Filename	Overview
src/components/tl/nccl/tl_nccl_coll.c	Adds native ncclAlltoAll support for NCCL 2.28.0+ with proper fallback to send/recv pattern for older versions

[greptile-apps]

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[Sergei-Lebedev]

/build

[janjust]

/build

[Sergei-Lebedev]

/build