function "pair_w_tokens_with_ground_truth_span" could not find similar answers

[dujiaxin]

I got lots of warnings from logger.warning("Can not find answer span from whitespace-tokenized context, there might be a problem") logger.warning("Could not find answer: '%s' vs. '%s'", a, b)

I checked the warnings and found that lot's of the answers should be matched. There were just some whitespace and Lower/Upper case issues. Maybe it can be fixed.