Remove support for passing strategy name to plugins

[awaelchli]

What does this PR do?

Part of #12521

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

Yes. The following is no longer supported:

Trainer(plugins="ddp_spawn")
Trainer(plugins="ddp_find_unused_parameters_false")
Trainer(plugins=DDPStrategy())

Instead, use:

Trainer(strategy="ddp_spawn")
Trainer(strategy="ddp_find_unused_parameters_false")
Trainer(strategy=DDPStrategy())

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Did you have fun?

I made sure I had fun coding 🙃

cc @justusschock @awaelchli @rohitgr7 @Borda @kaushikb11 @ninginthecloud

[rohitgr7]

still draft?

[awaelchli]

@rohitgr7 waiting to merge the other one first.