Convert preprocessInput functions to Operations and allow to call them from the preprocessing pipeline

[juliabeliaeva]

This PR aims at removing custom preprocessInput functions which had to be called after normal preprocessing and converting them into Operations and providing api to call these in the preprocessing pipeline.

[ermolenkodev]

Just for discussion. It can be not so obvious that the fileLoader created from Preprocessing will combine loading and preprocessing. We could consider splitting loading and preprocessing into two separate steps.

[juliabeliaeva]

Just for discussion. It can be not so obvious that the fileLoader created from Preprocessing will combine loading and preprocessing. We could consider splitting loading and preprocessing into two separate steps.

Agree. And the syntax ".fileLoader" combined with the preprocessing pipeline looks a bit weird.