revert tf 2.1

Author: previ

requirements_stub.txt

@@ -36,7 +36,7 @@ six==1.14.0
 swagger-spec-validator==2.3.1
 termcolor==1.1.0
 tinydb==3.12.1
-tensorflow==2.7.0
+tensorflow==2.1.0
 tensorflow_hub>=0.7.0
 urllib3==1.24.2
 Werkzeug==0.15.3