Fix squad answer start and end position
Previous version might miss some overlong answer start and end indices (which should be 0), and sometimes the start/end positions would be outside the model inputs.

Doc_start and doc_end are based on tokenized subword sequences while example.start_position and example.end_position are indices in original word-level, which are usually shorter than the subword sequences. Why not use tok_start_position and tok_end_position for comparison?
Thanks @cooelf, there was a PR merging `run_squad` and `run_squad2` that also fixed this issue.