Remove reference to DistributedDataParallel from parallel plugin teardown

[four4fish]

What does this PR do?

Parallel plugin should be generic and self-contained. Move reference to DistributedDataParallel from parallel teardown()

Subtask 1 for "remove reference to DistributedDataParallel from parallel plugin"

Tests:
python -m pytest -v tests/plugins/test_ddp_spawn_plugin.py passed
dev python -m pytest -v tests/plugins/test_ddp_plugin.py passed

Does your PR introduce any breaking changes? If yes, please list them.

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[codecov]

Codecov Report

Merging #8943 (59592cb) into master (53885af) will decrease coverage by 4%.
The diff coverage is 58%.

❗ Current head 59592cb differs from pull request most recent head ca53ba0. Consider uploading reports for the commit ca53ba0 to get more accurate results

@@           Coverage Diff           @@
##           master   #8943    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         176     176            
  Lines       14790   14673   -117     
=======================================
- Hits        13638   12894   -744     
- Misses       1152    1779   +627     

[tchaton]

LGTM ! But I believe we already have test for this.

[four4fish]

Remove the unit test as it's already covered. Will add more unit test if it's needed after #9096