Bump tensorflow from 2.1.2 to 2.5.2 in /elasticdl (#2534)

Bumps [tensorflow](https://github.com/tensorflow/tensorflow) from 2.1.2 to 2.5.2.
- [Release notes](https://github.com/tensorflow/tensorflow/releases)
- [Changelog](https://github.com/tensorflow/tensorflow/blob/master/RELEASE.md)
- [Commits](tensorflow/tensorflow@v2.1.2...v2.5.2)

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

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elasticdl/requirements.txt

@@ -3,6 +3,6 @@ docker==4.2.1
 pyrecordio>=0.0.6
 Cython
 odps
-tensorflow==2.1.2
+tensorflow==2.5.2
 psutil
 cloudpickle