Transformers: Add a BERT task

[martinjaggi]

Add BERT as a new task
we can reuse existing transformer code we already have for the translation NLP task from here:
#33

@guptaprkhr @jbcdnr @mpagli can you point us to the best code to start from, including the pre-processing pipeline? and later have a look at it here again so we can get a draft of the standard data-parallel training running?

we should define a very light goal on the BERT training loss at first, to have something to iterate on quickly.

MLperf for comparison currently only has tensorflow BERT