* Fix bug ranker: wrong lambda function
The zip function used in line 110 intends to choose the logits array to be the key for the lambda function while it should be the first/second logit of the logit array which corresponds to the classification label (has_answer)
* Use label 1 as has_answer label
* generic ranker (add if-cond for logits vector shape)
* remove test code
* remove test code...
* add two_logits test case for ranker module.
* complete the documentation of ranker, support rankers with 1 or 2 logits as output