feat(wandb): log in sync with Trainer step

[borisdayma]

What does this PR do?

Uses directly Trainer step to log metrics and keep it in sync with wandb step.

They were decoupled due to an issue on k-fold logging, where we use multiple Trainer for the same W&B run.

This created confusion for some users as wandb was then using global_step as a parameter to refer to Trainer step.
global_step is not used anymore and instead, we offset step when logging additional Trainer loops on the same W&B run.

Fixes #4113

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[codecov]

Codecov Report

Merging #4405 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #4405   +/-   ##
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  Coverage      93%     93%           
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  Files         111     111           
  Lines        8081    8081           
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  Hits         7489    7489           
  Misses        592     592           

[awaelchli]

nice