Filter certain paths from the beam search

[andrea-gasparini]

Hi everyone,

I have a use-case for which, based on some external context-based knowledge, I need to filter out certain paths from the beam search so that we both avoid to have them in the final output and we also let the beam search explore unconstrained paths. Since it doesn't seem to me that something similar is already planned or implemented I was wondering if after modifying the code for myself I could also open a Pull Request to add such a feature or it would not be of interest for the purposes of the library.

Anyway, thanks for the great work!