fix: Set tensorboard's global_step as the default wandb x-axis if sync_tensorboard=True

[ringohoffman]

What does this PR do?

There isn't a need to create trainer/global_step if you are syncing from tensorboard with sync_tensorboard=True.

Since I use TensorboardLogger as my main logger and sync it to wandb with WandbLogger(sync_tensorboard=True), in my case, WandbLogger.log_metrics is never called, and trainer/global_step is never updated. All of my wandb are shown as a single point by default, and I have to manually update my workspace/plots to use global_step.

global_step	trainer/global_step

It makes a lot more sense to just default to global_step as the x-axis if sync_tensorboard=True.

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📚 Documentation preview 📚: https://pytorch-lightning--20611.org.readthedocs.build/en/20611/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (71793c6) to head (c0de998).
Report is 113 commits behind head on master.

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HEAD has 102 uploads less than BASE

Flag	BASE (71793c6)	HEAD (c0de998)
cpu	48	24
lightning	37	18
pytest	26	0
python3.9	12	6
lightning_fabric	7	0
python3.11	12	6
python3.10	6	3
python3.12.7	18	9
gpu	2	0

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20611     +/-   ##
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- Coverage      88%      79%     -9%     
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  Files         267      264      -3     
  Lines       23369    23316     -53     
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- Hits        20478    18371   -2107     
- Misses       2891     4945   +2054     

[ringohoffman]

cc: @Borda

[ringohoffman]

Any thoughts on this?

[ringohoffman]

It looks like 2 tests timed out. Can they be retried?