Bump tensorflow from 2.1 to 2.4.0

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.1 to 2.4.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](https://github.com/tensorflow/tensorflow/compare/v2.1.0...v2.4.0)

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -11,7 +11,7 @@ protobuf
 PyYAML
 scipy
 six
-tensorflow==2.1
+tensorflow==2.4.0
 termcolor
 tqdm
 Werkzeug