Remove Dependency between Bart and LED (slow/fast)
# What does this PR do?

Removes the dependency between LED and Bart

## Who can review?
@sgugger 
@sgugger hopefully this does it?<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Just tried locally your PR and found the reason why we can't have a global copied from on the tokenizer (sorry I didn't spot it earlier): the `_pad` method is overwritten.
So instead we need to applied the copied from on each method (except `_pad`) and not copy the whole class.

Sorry about that!<|||||>> Just tried locally your PR and found the reason why we can't have a global copied from on the tokenizer (sorry I didn't spot it earlier): the `_pad` method is overwritten. So instead we need to applied the copied from on each method (except `_pad`) and not copy the whole class.
> 
> Sorry about that!

so basically, write those copy comments again on both the slow and fast tokenizers?<|||||>Yeah, sorry<|||||>> Yeah, sorry

Oh dont be, on it now :D, thanks for the quick reply!<|||||>@sgugger anything left?<|||||>btw, the `mask_token` method in the fast tokenizer, there is a method and a setter , but i have the same comment on both (Copied from `BartTokenizerFast.mask_token)` is that the right way or am i supposed to keep the one above the method only and ignore the setter's one?<|||||>thanks a lot for the quick replies, what else is left?<|||||>Should be good now, just waiting for all tests to pass :-)<|||||>oh finally, the green light