Add is_in and stop_at generation constraints to transformers integration

[rlouf]

We add two constraints to the transformers integration:

• The is_in keyword argument allows to restrict the generations within a particular set. This is achieved by dynamically computing a mask and stopping when one of the sequences has been generated;
• The stop_at keyword allows to stop the generation when a given sequence has been generated. This sequence is not returned.

Closes #107

TODO

• Generation can be stopped sooner than it currently is for is_in: we don't need to generate the entire sequence, but rather exclude all the other sequences
• The whole "constrained generation" part of the file could do with some refactoring. Code could be better structured and documented.