Add deprecation path for renamed training type plugins

[awaelchli]

What does this PR do?

Part of #10549

Adds a deprecation path for the training type plugins that we renamed in #11226.
Example: In 1.6 this usage will be deprecated

from pytorch_lightning.training_type import DDPPlugin
Trainer(strategy=DDPPlugin(find_unused_parameters=False))

and should be replaced with

from pytorch_lightning.strategies import DDPStrategy
Trainer(strategy=DDPStrategy(find_unused_parameters=False))

TODO:

• add tests for the deprecated classes
• add deprecation warning to the docstring

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

No, but after v1.8, when the old plugin aliases are removed, all imports will break.

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @tchaton @rohitgr7 @Borda

[carmocca]

This won't run at the moment because we only run ipu tests on the ipu test file

[awaelchli]

Do you think #11170 will land? I can drop the test case completely, move it to the ipu test file, or keep as is

[carmocca]

I don't know. I would move the test to test_ipu.py and add a comment here referencing it.

[tchaton]

LGTM !