Remove support for passing strategy strings to accelerator

[awaelchli]

What does this PR do?

Part of #12521

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

Yes. This is no longer supported:

Trainer(accelerator="ddp_spawn")
Trainer(accelerator="deepspeed")
Trainer(accelerator="ddp_cpu")

Instead, use the strategy argument:

Trainer(strategy="ddp_spawn")
Trainer(strategy="deepspeed")
Trainer(strategy="ddp", accelerator="cpu")

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @justusschock @kaushikb11 @awaelchli @ninginthecloud

[krshrimali]

Thanks, @awaelchli! 🚀 LGTM, just a minor comment.