Instable learning with SemiMarkov CRF

[urchade]

HI,

First, thank you for fixing #110 (@da03), the SemiCRF works better now, I was able to get good results on span extraction tasks. However, I still encounter a learning instability where the loss (neg logprob) gets negative after several steps (and the accuracy starts to drop). The same problem occurs with batch_size = 1. Below I put the learning curve (f1_score and log loss).

(Maybe the bug comes from the masking of spans where (length, length + span_with) and length + span_with > length, but I am not sure.)

Edit: I created a test and it seems that the masking is good. Maybe the log_prob computation or the to_parts function ?