Remove support for DDP2 strategy

[awaelchli]

What does this PR do?

Fixes #12584

Raises an error for users who subclass or use the DDP2Strategy/DDP2Plugin. Raises an error for everyone trying to use it with strategy="ddp2".

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

Yes. If you try this, you get an error:

trainer = Trainer(strategy="ddp2")

Since DDP2 is broken and unsupported on torch >= 1.9, you will need to downgrade Lightning to < 1.5 with torch < 1.9.
It is possible to instantiate the DDP2Strategy and pass it into the strategy argument, but it will still print a deprecation warning.

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Did you have fun?

I made sure I had fun coding 🙃

cc @tchaton @rohitgr7 @Borda @justusschock @awaelchli

[tchaton]

Neat !