Change keras dependency to a stable version

Author: rimolive

backend/src/apiserver/visualization/requirements.txt

@@ -73,8 +73,8 @@ joblib==0.14.1            # via scikit-learn, tensorflow-data-validation
 jsonschema==3.2.0         # via nbformat
 jupyter-client==5.3.5     # via -r -, ipykernel, notebook
 jupyter-core==4.9.1       # via jupyter-client, nbconvert, nbformat, notebook
-jupyterlab-widgets==1.0.2  # via ipywidgets
-keras-nightly==2.5.0.dev2021032900  # via tensorflow
+jupyterlab-widgets==1.0.2 # via ipywidgets
+keras==2.6.0              # via tensorflow
 keras-preprocessing==1.1.2  # via tensorflow
 libcst==0.3.23            # via google-cloud-bigquery-storage
 markdown==3.3.6           # via tensorboard