[Enhancement] Vocab List in Dataloader

[hzhwcmhf]

For implemention of #8 copynet, dataloader should change behaviours.

In our mind, there should be 3 vocab list:

• For model trainning, smallest. Only include words from train set. Call it set $V.
• For metric, bigger. The model will be evaluated on this vocab list, including words from train set and test set. Call it set $M. But almostly all models can't generate words from $V-$M, because they haven't seen these. Howerver, copyNet can gen words from $V-$M by copy mechanism. It's necessary to take these words into accounts when we implement metrics. $V-$M can be expressed as UNK token for some models. Dataloader have to tranlate them into a uniform distribution on $V-$M.
• The whole space of word, include not seen in all the data. Call it set $N. The words in $N-$M, we don't care about them, ignore them in evaluating models, as [BUG] bug in trim_index of dataloader #37 . $N-$M is the TRUE UNK.

Require:

• Change the behavior of dataloader, metric.

[hzhwcmhf]

Done