Trainer `strategy` argument is case sensitive

[ethanwharris]

🐛 Bug

The strategy argument is case sensistive so ddp works but DDP doesn't.

To Reproduce

strategy="DDP" gives an error that it's not in the registry...

Additional context

This probably applies to all things using the registry, may make sense to make it case-insensitive.

[kaushikb11]

Yup, we could make it case-insensitive.