deprecate flush_logs_every_n_steps on Trainer

[edward-io]

What does this PR do?

• Deprecates flush_logs_every_n_steps
• Moves flush_logs_every_n_steps init to individual loggers (only CSV logger has the ability to flush afaict)

Fixes #8991

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

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[edward-io]

I'm not sure why the doc tests are failing. There aren't any obvious error messages, only some warnings. Building them locally on a laptop works for me.

[awaelchli]

I think that's because you commited submodule changes in _notebooks. Make sure to remove that again.

[awaelchli]

TensorBoardLogger also has "flushing" implemented as part of save(). Can you add the argument there too? Save is what is called in the training loop when should_flush_logs=True. And on the summary writer for tensorboard there is also a flush method but we only call it in finalize(). So confusing. I guess it's good that this gets deprecated.

[codecov]

Codecov Report

Merging #9366 (5fa4296) into master (ec828b8) will decrease coverage by 4%.
The diff coverage is 100%.

❗ Current head 5fa4296 differs from pull request most recent head e619ce1. Consider uploading reports for the commit e619ce1 to get more accurate results

@@           Coverage Diff           @@
##           master   #9366    +/-   ##
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- Coverage      93%     89%    -4%     
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  Files         180     179     -1     
  Lines       15066   14933   -133     
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- Hits        13991   13261   -730     
- Misses       1075    1672   +597     

[rohitgr7]

LGTM!

[tchaton]

LGTM !