Better dummies (huggingface#15148)

* Better dummies

* See if this fixes the issue

* Fix quality

* Style

* Add doc for DummyObject

src/transformers/file_utils.py

@@ -831,6 +831,18 @@ def requires_backends(obj, backends):
         raise ImportError("".join([BACKENDS_MAPPING[backend][1].format(name) for backend in backends]))
 
 
+class DummyObject(type):
+    """
+    Metaclass for the dummy objects. Any class inheriting from it will return the ImportError generated by
+    `requires_backend` each time a user tries to access any method of that class.
+    """
+
+    def __getattr__(cls, key):
+        if key.startswith("_"):
+            return super().__getattr__(cls, key)
+        requires_backends(cls, cls._backends)
+
+
 def add_start_docstrings(*docstr):
     def docstring_decorator(fn):
         fn.__doc__ = "".join(docstr) + (fn.__doc__ if fn.__doc__ is not None else "")