Error messages for removed Trainer mixin methods

[awaelchli]

What does this PR do?

Follow up to #14888, #14887 in accordance to #15049.

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @justusschock @kaushikb11 @awaelchli @ananthsub @ninginthecloud @jjenniferdai @rohitgr7 @akihironitta

[otaj]

I like the idea of graveyard, but wouldn't it be neater to have a magic decorator, that would take as params what is deprecated, when it was deprecated and removed and what should be used instead? I feel like we're duplicating quite a lot of text, that could be stored within this one magic function. cc @carmocca

[carmocca]

@otaj You would need to be more specific with your proposal.

But in general, we don't care much about what's inside the graveyard (it's all dead code!)

In my PR, I did some hacking to create classes dynamically and write less code: https://github.com/Lightning-AI/lightning/pull/15061/files#diff-6ccf6181b48656c99d6106f8a2c2f98904cdc7f06155bdffd9b02ec5afa48c45R51 but I don't think we really need to care about writing the least possible lines of code.

So, if you have a specific suggestion, we can totally add it as long as it's self-contained inside the graveyard

[otaj]

@carmocca, perfectly fair, this was just an idea in my head. The argument, that it's all dead code, is perfectly valid., let's disregard my comment 😄 ⚡