title: precision , recall , f score when y_pred & y_true have different sizes
body: <p>I have two lists:
  "A" is a list of lists, each item containing list of predicted tags
  "B" is a list of lists, each item containing true tags</p>
  
  <p>I'm trying to calculate precision, recall, f score using scikit-learn in iteration, A[i] compared to B[i] but it says</p>
  
  <p>ValueError: Found input variables with inconsistent numbers of samples:[3, 2]</p>
  
  <p>I found some similar topics, but there was no answer <a href="https://stackoverflow.com/questions/37621442/computing-precision-and-recall-for-two-sets-of-keywords-in-nltk-and-scikit-for-s">Computing precision and recall for two sets of keywords in NLTK and Scikit for sets of different sizes</a></p>