[Feature] Optimize forward metadata collection across dp ranks

[jianzs]

Resubmit #1593

What this PR does / why we need it?

This PR introduces two optimizations for cases where data parallel size > 1:

1. Eliminates DP communication in set_forward_context
2. Implements HCCL for DP metadata communication, resulting in significant performance improvements for large DP configurations
   • Achieves ~20ms latency reduction with DP size of 64

Does this PR introduce any user-facing change?

no

How was this patch tested?

CI passed.

• vLLM version: v0.9.2
• vLLM main: vllm-project/vllm@b9a21e9

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 54.80%. Comparing base (33ef5dc) to head (2b0c3c1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1857   +/-   ##
=======================================
  Coverage   54.80%   54.80%           
=======================================
  Files          75       75           
  Lines        9327     9327           
=======================================
  Hits         5112     5112           
  Misses       4215     4215           

Flag	Coverage Δ
unittests	54.80% <ø> (ø)

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[Yikun]

Rebase here: f9dfde0