Help with implementing `dropout`

Hi everyone (again),

I wonder if there are any plans on adding dropout to `bpe` as described [here](https://arxiv.org/abs/1910.13267) (third page, Algorithm 1)? 

If not, to which method you'd assume it's simpler to add? Since backtracking tokenizer works slightly different compared to original BPE, I hadn't much success looking for easiest place to plug `dropout` myself.