This PR will introduce a `BiAttentionPoolingClassifier` as in [Attention is all you need](https://arxiv.org/abs/1706.03762) following the discussion with @sebastianruder in Teams.
I ran out of memory on my 1060 while testing the attention module, but was able to at least verify that it is functionally correct. Some changes might be required to ensure that the tensor passed to `self.layers` is of the right shape (but I'm not quite sure as of now).
I'll shift all the stuff to Collab for testing and see if it's any help.
aayush