Bump tensorflow-gpu from 1.13.1+nv to 2.4.0 in /python-ff

Bumps [tensorflow-gpu](https://github.com/tensorflow/tensorflow) from 1.13.1+nv 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/commits/v2.4.0)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

python-ff/requirements.txt

@@ -80,7 +80,7 @@ six==1.12.0
 syntax
 tensorboard==1.13.1
 tensorflow-estimator==1.13.0
-tensorflow-gpu==1.13.1+nv
+tensorflow-gpu==2.4.0
 termcolor==1.1.0
 terminado==0.8.2
 testpath==0.4.2