fix: use dist.reduce_scatter_tensor to avoid memory leak

[NeverRaR]

What this PR does / why we need it?

fix: use dist.reduce_scatter_tensor to avoid memory leak

Does this PR introduce any user-facing change?

How was this patch tested?

• vLLM version: v0.9.1
• vLLM main: vllm-project/vllm@5358cce

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 54.47%. Comparing base (c30ddb8) to head (ae879a4).
Report is 107 commits behind head on main.

Files with missing lines	Patch %	Lines
vllm_ascend/ops/fused_moe.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1688       +/-   ##
===========================================
+ Coverage   27.39%   54.47%   +27.08%     
===========================================
  Files          56       79       +23     
  Lines        6191     9927     +3736     
===========================================
+ Hits         1696     5408     +3712     
- Misses       4495     4519       +24     

Flag	Coverage Δ
unittests	54.47% <0.00%> (+27.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[ApsarasX]

duplicate of PR #1380

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.