Proposed refactor

Part of #9932
Related PR: #10548

Motivation

As we are introducing StrategyPlugin in favor of TrainingTypePlugin for this release.

@ananthsub raised some really good points for discussion regarding StrategyPlugin. Reference

Proposed solution

Three main points to be discussed here

• Drop Plugin from the name of the class
• It should be under pytorch_lightning/strategy directory
• Over time, these classes should be renamed as DDPStrategy and so on.

Would like your thoughts? @PyTorchLightning/core-contributors

cc @Borda @justusschock @awaelchli @akihironitta @kaushikb11 @ananthsub