fix nano UT (#5650)

docs/readthedocs/source/doc/Nano/Overview/known_issues.md

@@ -50,4 +50,10 @@ spec:
 
 ### **Nano keras multi-instance training currently does not suport tensorflow dataset.from_generators, numpy_function, py_function**
 
-Nano keras multi-instance training will serialize TensorFlow dataset object into a `graph.pb` file, which does not work with `dataset.from_generators`, `dataset.numpy_function`, `dataset.py_function` due to limitations in TensorFlow.
+Nano keras multi-instance training will serialize TensorFlow dataset object into a `graph.pb` file, which does not work with `dataset.from_generators`, `dataset.numpy_function`, `dataset.py_function` due to limitations in TensorFlow.
+
+## **Ray Issues**
+
+### **protobuf version error**
+
+Now `pip install ray[default]==1.11.0` will install `google-api-core==2.10.0`, which depends on `protobuf>=3.20.1`. However, nano depends on `protobuf==3.19.4`, so if we install `ray` after installing `bigdl-nano`, pip will reinstall `protobuf==4.21.5`, which causes error.