Updated batched seq2seq example

This is my version of pytorch seq2seq tutorial.

New features:

• validation data. Separate dataset for validation is used, this gives much more accurate picture of model training than just using training data
• refactored code:
  • using dataloaders
  • separate classes for data management
  • learner class for model creation and training
• data tokenization is multicored
• possibility to use pretrained wordvectors
• added teacher forcing

For more details, explanations see my article.

See example in example_seq2seq.ipynb. Example data is from IMDB dataset.

Inspiration and some of the code is from fastai and from the original tutorial.