Integrate sentence transformers into benchmarks

[Timoeller]

Would be nice to compare the new sentencetransformers models, especially the
SentenceTransformer('nq-distilbert-base-v1') and how it compares against DPR finetuned on NQ.

Made sure to use cosine similarity (so had to use ES doc store).
Documents for Sentence Transformers should be a list of [title, text] lists.

Preliminary results do not look better than DPR:
for 100k docs we have mAP of 82.7 SBert vs 86.5 DPR
Details:

100k reference docs
'retriever': 'sentence_transformers', 'doc_store': 'elasticsearch', 'n_docs': 100000, 
'n_queries': 5637, 'retrieve_time': 1288.7816320069996, 
'queries_per_second': 4.37389846348259, 'seconds_per_query': 0.22862899272786938, 
'recall': 93.79102359411034, 'map@10': 82.74644426986096, 
'top_k': 10, 'date_time': datetime.datetime(2021, 2, 17, 19, 29, 25, 961023), 'error': None}


500k reference docs
{'retriever': 'sentence_transformers', 'doc_store': 'elasticsearch', 'n_docs': 500000, 
'n_queries': 5637, 'retrieve_time': 5642.244277841075, 
'queries_per_second': 0.9990705333582115, 'seconds_per_query': 1.0009303313537476, 
'recall': 89.62213943587014, 'map@10': 76.49362488771762, 
'top_k': 10, 'date_time': datetime.datetime(2021, 2, 17, 23, 44, 11, 757607), 'error': None}

[Timoeller]

I checked the data and it seems ok.

No answers as well as long answers are removed from NQ dev set.
We put in 100 word passages containing an answer string as positive passages.

Lets revert the config and fix the mypy bug, then we are ready to merge from my side. What do you think @brandenchan ?

[brandenchan]

This all looks good to me. Just one very tiny comment. Its ready for merge as far as I'm concerned once the mypy bug is fixed and the config is reverted.

[Timoeller]

Hey @brandenchan I fixed mypy and reverted the config.

Could you double check, also with the conflicting docs files, so that we can merge?

[brandenchan]

Hey @brandenchan I fixed mypy and reverted the config.

Could you double check, also with the conflicting docs files, so that we can merge?

Ok nice! The conflicting docs seem to be because new arguments were added to functions in master, we updated doc strings and the api documentation was regenerated. I would in each conflict case take the change from master

[brandenchan]

Once documentation conflicts are resolved, this PR is ready for merge