Fix how we detect the TF package (#24255)

* Fix how we detect the TF package

* Add a comment as a talisman warding against future harm

* Actually put the comment in the right place

src/transformers/utils/import_utils.py

@@ -147,7 +147,9 @@ if FORCE_TF_AVAILABLE in ENV_VARS_TRUE_VALUES:
     _tf_available = True
 else:
     if USE_TF in ENV_VARS_TRUE_AND_AUTO_VALUES and USE_TORCH not in ENV_VARS_TRUE_VALUES:
-        _tf_available = _is_package_available("tensorflow")
+        # Note: _is_package_available("tensorflow") fails for tensorflow-cpu. Please test any changes to the line below
+        # with tensorflow-cpu to make sure it still works!
+        _tf_available = importlib.util.find_spec("tensorflow") is not None
         if _tf_available:
             candidates = (
                 "tensorflow",