f1-score evaluation

[p16i]

Hi,

While working on #8, it seems to me that the evaluation of f-score is based on flatten true and pred labels. For example, given 2 samples whose lengths are 7 and 20. The current code flatten the labels to shape (27,) and compute the score. However, I think it could overestimate the value.

To illustrate, I've made a notebook using random data. You can see in there that the avg f-score is slightly lower than the f-score from the flatten data.

Looking forward to your thought on this.