Add PredictLoop

[tchaton]

What does this PR do?

This PR adds PredictLoop.
Currently the trainer.predict was using EvaluationLoop with trainer.testing.
Now, it uses its own loop and remove the EvaluationLoop dependency to predict.

Fixes #5475

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[carmocca]

Minor comments. LGTM

[SeanNaren]

Really nice and I like the description of the trainer flow, this should definitely make its way to the docs for users if possible!

[tchaton]

Really nice and I like the description of the trainer flow, this should definitely make its way to the docs for users if possible!

Thanks. I was thinking to make a cleaner for the documentation.

Best,
T.C