Add-warning-tokenizer
# What does this PR do?
Adds a warning when the user wants to use a fast tokenizer but it doesn't exist. Should help with #20817
_The documentation is not available anymore as the PR was closed or merged._<|||||>Should I just change the argument? 
I'm in favor of raising an issue rather than a warning here, WDYT? <|||||>No I was talking about the docstring, but this is actually addressed by another PR. We can't suddenly raise an error for this behavior as it would be breaking.