Change import pattern in TM

[SkafteNicki]

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What does this PR do?

Fixes #459
Changes the import pattern for metrics requiring third-party packages from

from torchmetrics import FID

to

from torchmetrics.image.fid import FID

the recommended import path will be stated in the example belonging to each metric.

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[ananthsub]

Thanks @SkafteNicki !

[Borda]

pls provide justification of the speed-up
this is a breaking change without any back compatibility!

[Borda]

@SkafteNicki lets follow with #459 (comment)

[SkafteNicki]

@Borda alright lets do it, but we should probably state this somewhere in the documentation?

[Borda]

@SkafteNicki that is an interesting failer, that shall happen with the original PR not here...
the case is that MAP is asking for some TorchVision version which is not minimal for TM 🐰