Refactoring checkpoint names for multiple models
Hi, @sgugger reupload without datasets dir and added tf_modeling files, removed extra decorator in distilbert.
Linked to #10193, this PR refactors the checkpoint names in one private constant. 

one note: longformer_tf has two checkpoints "allenai/longformer-base-4096" & "allenai/longformer-large-4096-finetuned-triviaqa". I set the checkpoint constant to "allenai/longformer-base-4096" and left the one decorator with "allenai/longformer-large-4096-finetuned-triviaqa".

Fixes #10193


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...The test pasts on my local machine, I ran make test, style, quality, fixup. I dont know why this failed..<|||||>I just rebased the PR to ensure that the tests pass. We'll merge if all is green!<|||||>Thanks guys