Standalone Lite: Accelerators

[awaelchli]

What does this PR do?

Adds the Accelerator base class and CPU, TPU, CUDA, MPS implementations.

• The base class has minor changes: 1) setup(Trainer) removed 2) setup_environment renamed to init_device
• PL's Accelerator base class now subclasses Lite's Accelerator class.
• Later on, PL can refactor their usage in strategies to completely rely on the implementations in Lite. The small blocker here currently is the usage of the trainer reference.

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Did you have fun?

I made sure I had fun coding 🙃

[carmocca]

LGTM. Previous unresolved comments about moving implementations can be TODO(lite)s

[tchaton]

LGTM !