[Reformer] automate axial_pos_shape
This PR automates the calculation of the axial_pos_shape.
checked that every combination of 2**n works using this sheet: https://docs.google.com/spreadsheets/d/19gnP1ve2fT2F59LNiky44SPpmtmOegfIlk5_3OFXjyU/edit?usp=sharing

@patrickvonplaten 

Hi @flozi00,

Thanks for the PR! To be honest I don't think we should merge this. A couple of reasons:

1. In contrast to `num_buckets` which is said in the paper to be always around ~ `2 * sequence_length / chunk_length` `axial_pos_shape` can be freely set by the user. 
2. We are trying to add as little automatic settings that are not visible to the user as possible (also @thomwolf here). The reason is that it can later lead to errors that are hard to understand for the user. In this case, I don't think the user should use AxialPositionEmbeddings before having read the docs and understood how it works. Automatically setting the `num_buckets` is already suboptimal in this sense. <|||||>BTW, I just answered your email - sorry I forgot about this