Modified PR about pre/post-padding, supports masking, index input (no rows), pickled char map

[pinesnow72]

I modified followings:

• add an option for selecting pre or post-padding
• add mask support to ContextualizedEmbedding
• modified forward/backward index input by removing rows (sentence indices) and using tf.gather_nd with batch_dims=1
• add to script to extract charmap (char-to-int dictionary) from Flair embeddings and store it as a pickle
• use pickled charmap instead of char_to_int.py

[ydovzhenko]

Ok thanks! Give me a day to review carefully.

So the word is, Travis CI is not reliable for testing anymore. I'm working on a PR to switch from Travis CI to github actions, will also take a day or two. Then the checks on your PR will run. Stay tuned:-)

[ydovzhenko]

Hey, my bad for not getting back to you - I was behind on a deadline. I'll review within a week from now. I have not forgotten:-) Thank you for contributing!