Add allennlp components

[mmourafiq]

Use case

Allennlp has a great docker/cli interface for performing several operations: https://demo.allennlp.org/reading-comprehension

We should add components for all operations to allows users to use them easily, this can be also a very good set of tutorials for new users to get a quick sense of how they can put their models and internal knowledge to use.

Feature description

Allennlp already has a docker image so we just need to add components with inputs/outputs definition based on the cli. Users can use polyaxon run --hub allennlp:train -P...