Standalone Lite: Move Lite classes

[awaelchli]

What does this PR do?

Follow up to #14726.
This PR should be REBASE MERGED

Moves the LightningLite class and wrappers to the standalone package.
There are two commits:

• Moving the files/classes to the lite package.
• Add backward compatibility: PL has a subclass implementation in src/pytorch_lightning/lite with:
  • Conversion logic that turns gpus=x and tpu_cores=x into devices=x
  • Conversion logic that turns plugins=PLPrecisionPlugin into plugins=LitePrecisionPlugin
  • Conversion logic that turns strategy=PLStrategy into strategy=LiteStrategy

Please review the two commits separately.

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @justusschock @awaelchli @rohitgr7 @carmocca @akihironitta

[tchaton]

LGTM !