enable ffcv with PL

[williamFalcon]

A team introduced ffcv which does optimizations in the dataloader level.
As long as it can be a drop in replacement for dataloader it should add the benefits to any PyTorch and Lightning script.

ie:

# current
data = Dataloader(...)

trainer.fit(..., data)

# 10x faster with ffcv
data = FFCVDataloader

trainer.fit(..., data)

Let's make sure when users want to get the benefits of both FFCV and PL they can do that!
cc @Borda @rohitgr7 @Felonious-Spellfire @justusschock @awaelchli @ninginthecloud @otaj @tchaton @carmocca

[tchaton]

@ethanwharris This could be built-in within Flash.

[paantya]

libffcv/ffcv#72

[justusschock]

This is related to #10696.

When we support arbitrary iterables, ffcv should be supported automatically. Maybe not all features we support with basic torch loaders (haven't looked at ffcv details so far), but definitely usable for training.