replication results for MSMARCO document and CovidQA.

[qguo96]

Finally, replication on Colab through ssh is successful with K80 ....

Colab Environment:
OS: Ubuntu-18.04.3 LTS
Java: 11.0.8
Python: 3.6.9

results are shown below:

• experiments-msmarco-document(identical with the documents):
  Re-Ranking with monoT5:
  First half:

Second half:

• experiments-CovidQA(identical with the documents):
  Re-Ranking with Random:
  NL Question:
  

  Keyword Query:
  

  Re-Ranking with BM25:
  NL Question:
  

  Keyword Query:
  

  Re-Ranking with monoT5:
  NL Question:
  

  Keyword Query:
  

[ronakice]

LGTM! Thanks for adding this