Allow BatchEncoding to be initialized empty. (#4316)

* Allow BatchEncoding to be initialized empty.

This is required by recent changes introduced in TF 2.2.

* Attempt to unpin Tensorflow to 2.2 with the previous commit.

setup.py

@@ -67,8 +67,8 @@ extras = {}
 
 extras["mecab"] = ["mecab-python3"]
 extras["sklearn"] = ["scikit-learn"]
-extras["tf"] = ["tensorflow <= 2.1"]
-extras["tf-cpu"] = ["tensorflow-cpu  <= 2.1"]
+extras["tf"] = ["tensorflow"]
+extras["tf-cpu"] = ["tensorflow-cpu"]
 extras["torch"] = ["torch"]
 
 extras["serving"] = ["pydantic", "uvicorn", "fastapi", "starlette"]

src/transformers/tokenization_utils.py

@@ -173,7 +173,11 @@ class BatchEncoding(UserDict):
 
     """
 
-    def __init__(self, data: Dict[str, Any], encoding: Optional[Union[EncodingFast, Sequence[EncodingFast]]] = None):
+    def __init__(
+        self,
+        data: Optional[Dict[str, Any]] = None,
+        encoding: Optional[Union[EncodingFast, Sequence[EncodingFast]]] = None,
+    ):
         super().__init__(data)
 
         if isinstance(encoding, EncodingFast):