[Draft PR] allow logging on_train_end

[Al3xDo]

What does this PR do?

Fixes #15143

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[carmocca]

Logging on_train_end cannot be supported. The linked issue is still open for the suggestion of

Do you think it is meaningful to add a hint to our error messages that say "it is not allowed to self.log in x", where we would recommend "you can alternatively log with logger.experiment..."?

As proposed in #15143 (comment)

[Al3xDo]

Hi @carmocca, so I have to wait for the final decision (the error messages) on the linked issue and then work on it, right?

[carmocca]

You can go ahead and work on adding the hint. That would close the issue

[Al3xDo]

Thank you for your support, I will create another PR now