In Ascend NPU, stop workers together with its children processes

[majieyue]

What changes were proposed in this pull request?

Be adapt to Ascend NPU cases, to stop workers and make sure no remaining processes are using NPU

Why are the changes needed?

In Ascend NPU, workers will fork many child processes, and we need to clear all of them

Does this PR introduce any user-facing change?

NO

How was this patch tested?

UT

[codecov]

Codecov Report

Attention: Patch coverage is 95.34884% with 2 lines in your changes missing coverage. Please review.

Project coverage is 80.57%. Comparing base (6764a09) to head (c2e69a4).
Report is 26 commits behind head on master.

Files with missing lines	Patch %	Lines
dlrover/python/elastic_agent/torch/training.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1284      +/-   ##
==========================================
+ Coverage   80.41%   80.57%   +0.15%     
==========================================
  Files         219      219              
  Lines       20126    20167      +41     
==========================================
+ Hits        16185    16249      +64     
+ Misses       3941     3918      -23     

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