Set DistributedSampler seed if seed_everything was called

[awaelchli]

What does this PR do?

Fixes #7016

By default our DistributedSampler is initialized with seed 0. This is fine, we need to set a seed there for distributed training because the samples need to be split between the GPUs/nodes. There is however currently no way to customize it unless the user supplies their own DistributedSampler. We can automate that by including the seed from seed_everything, if user calls it.

Note: For everyone that is using seed_everything(x) today, this feature will be affect reproducibility because the seed in the sampler will be set to x instead of 0. If this is undesired, we need to add a flag to seed_everything that can turn this off.

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[codecov]

Codecov Report

Merging #7024 (d18555f) into master (402a258) will decrease coverage by 3%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #7024     +/-   ##
========================================
- Coverage      92%     89%     -3%     
========================================
  Files         194     197      +3     
  Lines       12403   13429   +1026     
========================================
+ Hits        11436   11925    +489     
- Misses        967    1504    +537     

[tchaton]

LGTM !