Refactor group_concurrent_contiguous in NIXL

[yuan-luo]

Motivation

Vectorize group_concurrent_contiguous() in NIXL to accelerate KVCache transfer.

Modifications

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
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[yuan-luo]

This PR is similar with #5834.