removed dependency on bart tokenizer(slow/fast) LED
# What does this PR do?

removes the dependency between LED tokenizer and Bart's (slow version)

Fixes # (issue)

follows hugging's face philosophy of Do Repeat Yourself


## Who can review?
@sgugger 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Thanks a lot for the feedback! ill fix everything and open a brand new one. 