Add logger state dumping and restoring

[ashleve]

What does this PR do?

Implements state dumping and resuming mechanism for loggers. Implements state_dict and load_state_dict() for tensorboard.

Related to #5342

This PR is a reimplementation of https://github.com/PyTorchLightning/pytorch-lightning/pull/6361

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Did you have fun?

Always 🙃

[ashleve]

Currently resuming tensorboard resets the logged step number https://github.com/PyTorchLightning/pytorch-lightning/issues/12274

[ashleve]

Could anyone help with the tests?

I can't explain why the pickling is failing.

Also not sure how to fix the hooks related tests from tests/models/test_hooks.py.

[ashleve]

@carmocca Thank you for your help! I fixed the tests accordingly

[ashleve]

@awaelchli @carmocca Please review the changes when you find the time

[Borda]

@carmocca, how is it going here? :otter:

[carmocca]

@awaelchli It would be nice to finish it. Since you are familiar with the state key, do you want to take it? Otherwise I will. Not a high priority of course.

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⚠️ GitGuardian has uncovered 2 secrets following the scan of your pull request.

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🔎 Detected hardcoded secrets in your pull request

GitGuardian id	Secret	Commit	Filename
-	Generic High Entropy Secret	78fa3afdfbf964c19b4b2d36b91560698aa83178	tests/tests_app/utilities/test_login.py	View secret
-	Base64 Basic Authentication	78fa3afdfbf964c19b4b2d36b91560698aa83178	tests/tests_app/utilities/test_login.py	View secret

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