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 #6361

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Did you have fun?

Always 🙃

[ashleve]

Currently resuming tensorboard resets the logged step number #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.

[carmocca]

Hi @ashleve!

I can't explain why the pickling is failing.

Several existing tests pass mocks as logger instances for simplicity. Since this PR adds support for serializing their state, the logger.state_dict() of a mock logger returns another mock wich cannot be serialized. The existing tests will need to be updated to not use mocks or you will have to do: mock_logger.state_dict.return_value = {}

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

These tests check for the order of all hook calls, including their arguments. You probably need to update the "saved_ckpt" argument to include a logger key: https://github.com/PyTorchLightning/pytorch-lightning/blob/fbd887df9d487da4c57d884e01b3401af140b1bc/tests/models/test_hooks.py#L494-L503

I will assign Adrian as the main reviewer of the PR, but you can also ask me anything if you need support. Either here or in Slack.

Thank you for your work!

[Borda]

@carmocca, how is it going here? 🦦

[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.

[gitguardian]

⚠️ 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	78fa3af	tests/tests_app/utilities/test_login.py	View secret
-	Base64 Basic Authentication	78fa3af	tests/tests_app/utilities/test_login.py	View secret

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