Fix logic to check for spawn in worker_check

[rohitgr7]

What does this PR do?

Checking for spawn using this using_spawn = self.accelerator_connector.distributed_backend == "ddp_spawn" isn't
correct here because we turn of spawn if num_processes < 2 which is the case the test was running on and was still raising the warning which it shouldn't:
https://github.com/PyTorchLightning/pytorch-lightning/blob/4610fddb19d502e534b5c5d77c3dfd6f2e5359a5/pytorch_lightning/trainer/connectors/accelerator_connector.py#L790-L800

this PR just fixes that

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

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[rohitgr7]

the issue for failing test is that it's now actually running spawn process and we need to catch some warnings. have tried everything, but couldn't find a way.. feel free to commit.. any help would be appreciated :) thank you

[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[codecov]

Codecov Report

Merging #9902 (22cceb4) into master (7b4df7b) will decrease coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #9902   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         179     179           
  Lines       15839   15825   -14     
======================================
- Hits        14679   14661   -18     
- Misses       1160    1164    +4     

[tchaton]

LGTM !