Add env switch for AITER custom all-reduce

[apinge]

Motivation

This PR ports the feature of SGLANG_USE_AITER_NEW_CA from zejunchen-zejun#229

Modifications

Accuracy Tests

Speed Tests and Profiling

Checklist

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[Copilot]

Pull request overview

Ports the SGLANG_USE_AITER_NEW_CA feature into SGLang by wiring an environment-controlled switch into the custom all-reduce (“ca”) execution path in the distributed parallel state.

Changes:

• Adds SGLANG_USE_AITER_NEW_CA-driven behavior by passing use_new=... into the custom all-reduce call.
• Imports get_bool_env_var in parallel_state.py to read the new env switch.

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