Seed all workers when using DDP

[senarvi]

What does this PR do?

When seed_everything(workers=True) is called, it will set the environment variable PL_SEED_WORKERS=1. Consequently Trainer will set the worker_init_fn for dataloaders to pl_worker_init_function. However, this did not work with plugins that call reset_seed(), since that function calls seed_everything() with the default argument workers=False. This bug fix changes seed_everything() so that it will read the PL_SEED_WORKERS environment variable and pass the correct value for workers.

Fixes #7937

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[senarvi]

@awaelchli feel free to suggest better log messages.

[codecov]

Codecov Report

Merging #7942 (ee0a16b) into master (20a5e09) will decrease coverage by 5%.
The diff coverage is 67%.

@@           Coverage Diff           @@
##           master   #7942    +/-   ##
=======================================
- Coverage      92%     87%    -5%     
=======================================
  Files         203     203            
  Lines       13129   13131     +2     
=======================================
- Hits        12017   11399   -618     
- Misses       1112    1732   +620     

[tchaton]

LGMT. Great catch !