Rename the TrainingTypePlugin base to Strategy

[awaelchli]

What does this PR do?

Renames the base class to Strategy.
The next PRs will cover all related usages:

• Renaming subclasses, e.g., DDPPlugin -> DDPStrategy
• module training_type -> strategy
• Registry: Rename TrainingTypePluginsRegistry to StrategyRegistry #11122
• other variable names across the project

Part of #10549

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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 @Borda @justusschock @awaelchli @akihironitta

[github-actions]

Build Error! No Linked Issue found. Please link an issue or mention it in the body using #<issue_id>

[four4fish]

I marked this as breaking change as user who extend TrainingTypePlugin will need to update their class inheritance

[tchaton]

LGTM !

[kaushikb11]

This is a heavy breaking change on the import level itself for users who extend TrainingTypePlugin. Can’t we prevent it by introducing strategy.py file in the PR?

[justusschock]

@kaushikb11 But then we only delay the breaking changes. I think we could introduce a training_type_plugin file for BC, but I really think we should go with these changes as they were considered to be experimental. And since we want to have it stable for 1.6, we cannot delay the BC beyond this

[awaelchli]

@kaushikb11 To prevent a huge PR, I would prefer to go step by step by renaming parts individually. Once we move the files we can still add a simple reroute/deprecation path for mapping the training_type module to strategy