Allow editing sentences in between

[SebastinSanty]

Currently, we are performing beam search based on the inputs at the end. However, there should be a method to change inputs in between, so that the suggestion can be made based on the prefix and suffix.

Relevant Literature:

1. Fast Lexically Constrained Decoding with Dynamic Beam Allocation for Neural Machine Translation
2. Lexically Constrained Decoding for Sequence Generation Using Grid Beam Search
3. Guided Open Vocabulary Image Captioning with Constrained Beam Search