[QA examples] fix inconsistent tokenization in _improve_answer_span 
Basically, I am trying to fix the QA preprocessing steps when using BPE-based tokenizers (Roberta, Bart, Longformer)

The prior _improve_answer_span has a bug because the way it tokenizes the answer_text is different from the way context is handled. So when the original context is '1987', and the old input ans_span is ['Ġ1987', ','], then the ',' won't be removed.

To fix these cases I use two cleaned forms built by either including prefix_space or not. If a sub_answer_span matches either one of them we'll improve the answer_span. The first form [line 42] is used to handle things with leading punctuations, e.g., orig_answer_context=="73 million" and ans_span is ["Ġ$", "73", "Ġmillion"] (should be cleaned to  ["73", "Ġmillion"] ). The second form is used to handle things with ending punctuations, e.g., the '1987' example mentioned above.

@LysandreJik Not sure who I should ping to review this :) btw I am not sure what's the issue with the code quality here. <|||||>Hello, we're in the midst of deprecating that method and shifting the methods to `datasets` instead. Could you hold on until we have the updated approach, which should land sometimes within the next ~2 weeks?

This current method is not tested, so ensuring that a bug fix isn't creating all sorts of other bugs takes us a lot of time, so we're not going to do that here as this method will be deprecated very soon.<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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