build(deps): bump tensorflow-gpu from 2.0.0 to 2.5.1

Bumps [tensorflow-gpu](https://github.com/tensorflow/tensorflow) from 2.0.0 to 2.5.1.
- [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/v2.0.0...v2.5.1)

---
updated-dependencies:
- dependency-name: tensorflow-gpu
  dependency-type: direct:production
...

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

requirements.txt

@@ -1,4 +1,4 @@
 # XXX calamari 1.0.1 has bug (Early stopping frequency not honored #146), use
 # this git version:
 https://github.com/Calamari-OCR/calamari/archive/a4fa9d61.tar.gz
-tensorflow-gpu==2.0.0
+tensorflow-gpu==2.5.1