Fix allgather ops inside cuda graphs

[nvcastet]

Fixes #3424

TLDR: sglang uses pynccl or their customalllreduce instead of pytorch ProcessGroup when graph capturing. Allgather in sglang code base directly uses pytorch allgather instead of the sglang abstraction to decide which implementation to pick. Allgather is used in DP-attention and also to gather logits across TP dim. The fix is to perform the allgather via the abstraction so that the same NCCL communicator won't be used inside and outside graph captures.

Motivation

Modifications

Checklist

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