Potential bug in BM25 implementation

[IvanStelmakh]

When BM25 is executed with both use_abstract and use_title are set as true, a division by zero error occurs in line 47 of https://github.com/openreview/openreview-expertise/blob/master/expertise/models/bm25/bm25.py

This is because in lines 33-42 of the same file self.title_corpus list is not updated when use_abstract is set to true, irrespective of whether use_title is set to true or false.

Thus, if both use_abstract and use_title are set to true, in line 47 we have an empty list in the self.title_corpus which results into an error.

[carlosmondra]

Thanks! I will look into this