Fast transformers import part 1 (#9441)

* Don't import libs to check they are available

* Don't import integrations at init

* Add importlib_metdata to deps

* Remove old vars references

* Avoid syntax error

* Adapt testing utils

* Try to appease torchhub

* Add dependency

* Remove more private variables

* Fix typo

* Another typo

* Refine the tf availability test

.github/workflows/github-torch-hub.yml

@@ -33,7 +33,7 @@ jobs:
       run: |
         pip install --upgrade pip
         pip install torch
-        pip install numpy filelock protobuf requests tqdm regex sentencepiece sacremoses tokenizers packaging
+        pip install numpy filelock protobuf requests tqdm regex sentencepiece sacremoses tokenizers packaging importlib_metadata
 
     - name: Torch hub list
       run: |