New Feature: Best-First Beam Search
# 🚀 Feature request

<!-- A clear and concise description of the feature proposal.
     Please provide a link to the paper and code in case they exist. -->

Best-First Beam Search is a faster beam search algorithm for decoding.
https://arxiv.org/abs/2007.03909 (By Clara Meister, Tim Vieira, Ryan Cotterell)

## Motivation

This technique can be essential for production-purpose inference and of great interest to our users. It would be a good idea to integrate Best-First Beam Search to Hugging Face transformers (for GPT, BART, T5, etc.).
The first step would be refactoring our current implementation with the priority queue. I am on it.<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
