[Question] Pegasus tokenizer
@sshleifer - sorry to ping you here on this. Would be amazing if you find some time to explain the Pegasus tokenizer a bit.

A couple of things I don't understand:

- In the official Pegasus Tokenizer and from reading the paper it seems that exactly 2 mask tokens are necessary.
    See https://github.com/google-research/pegasus/blob/master/pegasus/ops/pretrain_parsing_ops.cc#L66
   a) ID=2 seems to correspond to the sentence mask token, called `[MASK_1]` and 
   b) ID=3 seems to correspond to the word mask token, called `[MASK_2]`

=> Why don't we have `[MASK_1]` and `[MASK_2]` tokens in the tokenizer's special tokens? I would actually add them at the id's 2 and 3 instead of having `unk_2` and `unk_3` there. Wdyt?

- Why do we call the tokens unk_2 - unk_104 ? Why unk? And why aren't those part of the `special_tokens_map` - is this on purpose? 

- Why does Pegasus inherit from the Reformer Tokenizer -> I don't really see what they have in common...

Would be awesome if you could take 10min to reply :-) 
No problem at all!

+ The inheritance is just for the purpose of not duplicating code.
+ You can change to whatever unk you would like, i wasn't at all careful about this stuff since i didn't try to replicate/test pre-training, just fine-tuning and generation.
+ Your changes sound like obvious low risk improvements. 
+ I don't know whether standard mask-filling will work for integration testing purposes, given the seq2seq pre-training objective .