Save model every n_steps

[yurakuratov]

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What problem are we trying to solve?:
I wan't to train a model and save its checkpoint for every n steps.
Such cases occur when there is no validation set or metric is not defined and model is evaluated later and separately from training.

How can we solve it?:
Add save_every_n_steps parameter to NNTrainer

Are there other issues that block this solution?:

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