Simplify working with optimizer variables

[juliabeliaeva]

This PR deals with optimizer variables created by the createSlot function. Instead of creating variables in the Optimizer#createSlots, putting them to the special map only to retrieve in the applyGradients, which is called directly after, variables are created immediately before usage.

[zaleslaw]

It's an interesting proposal, but I need a time to estimate the power of this refactoring, give me a couple of days here to trace all the code routes in refactored optimizers

[juliabeliaeva]

@zaleslaw I don't think this refactoring should be included into the 0.4 so there is really no rush with the review.

[zaleslaw]

If you suppose to postpone it, I suggest marking it with the 0.5 milestone