Avoid raising the sampler warning if num_replicas=1

[rohitgr7]

What does this PR do?

Dont' raise a warning if devices=1 already and ddp is still used.

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

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cc @justusschock @awaelchli @ninginthecloud @rohitgr7 @otaj

[codecov[bot]]

Codecov Report

Merging #14097 (91c5ced) into master (45a10a1) will decrease coverage by 3%. The diff coverage is 100%.

:exclamation: Current head 91c5ced differs from pull request most recent head 752c6d4. Consider uploading reports for the commit 752c6d4 to get more accurate results

@@            Coverage Diff             @@
##           master   #14097      +/-   ##
==========================================
- Coverage      79%      76%      -3%     
==========================================
  Files         106      326     +220     
  Lines        7066    26600   +19534     
==========================================
+ Hits         5592    20235   +14643     
- Misses       1474     6365    +4891     

[awaelchli]

@rohitgr7 We used to fall back to single device when strategy="ddp" and devices=1. We wouldn't even cross this code path since no distributed sampler is required in this case. This must have been lost in the accelerator connector "refactor".