Put all models in the constants
# What does this PR do?

It was impossible to use all pretrained checkpoints in the tapas tokenzier file because they were not in the constants of the file. This PR fixes that.
An alternative is to remove all content, but the previous state where one variable as some keys but the others not leads to failures.<|||||>Yes, we really need to remove all of this @LysandreJik @thomwolf <|||||>We do, but right now we still use them in the tests<|||||>It was a bit confusing to me which to add in the constants and which not.. 😅 <|||||>@sgugger one more small fix you can add is in tapas.rst under "Usage: inference", there should be one more space:

![docs](https://user-images.githubusercontent.com/48327001/102513749-e6b22b00-408b-11eb-9adb-060108f1b2e9.PNG)

(Sorry, if I see more things I make a new PR myself :p)<|||||>Pushing your fix on master directly Niels