Speed up tokenization by caching all_special_ids
Author: yashneeva
Pull request: yes

Also makes all_special_ids a set instead of list to speed up the "token in all_special_ids" call. In our tests these two changes made that line over 1000x faster, leading to a noticeable improvement in overall tokenization speeds.

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Comments:
Hi @yashneeva ,

Thank you very much for your proposal! Do you have some time to look at the failed tests? :hugs: 

> Thank you very much for your proposal! Do you have some time to look at the failed tests? 🤗

Hi sorry, was going to look at them before requesting review. Will try and take some time out today :)

Closing because I realized that making this work with the add_special_tokens function will require more work, and I don't have the bandwidth right now. Will look into it later if I free up :) Sorry about that!

My proposed fix would be to save the value of all_special_tokens and all_special_ids after the first computation, and update that every time add_special_tokens is called. For context, here is a screenshot of a pprof showing how much of the time in decode is being spent in the two all_special_ids calls (>80%).
<img width="658" alt="Screen Shot 2022-09-14 at 11 12 42 PM" src="https://user-images.githubusercontent.com/87332554/190327867-5d5123ca-3360-4f66-bf4e-1597129d76fe.png">