Bump tensorflow-gpu from 1.15.0 to 2.4.0

Bumps [tensorflow-gpu](https://github.com/tensorflow/tensorflow) from 1.15.0 to 2.4.0.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v1.15.0...v2.4.0)

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

requirements_specific.txt

@@ -29,7 +29,7 @@ scipy==1.3.3
 six==1.13.0
 tensorboard==1.15.0
 tensorflow-estimator==1.15.1
-tensorflow-gpu==1.15.0
+tensorflow-gpu==2.4.0
 termcolor==1.1.0
 urllib3==1.25.8
 Werkzeug==0.16.0