Remove ONNX from requirements.txt (#4073)

* Avoid installing ONNX package on aarch64

* Removed onnx from requirements

* Add note in backend.py

onnxruntime/python/backend/backend.py

@@ -21,6 +21,7 @@ class OnnxRuntimeBackend(Backend):
     multiple runtimes with the same API.
     `Importing models from ONNX to Caffe2 <https://github.com/onnx/tutorials/blob/master/tutorials/OnnxCaffe2Import.ipynb>`_
     shows how to use *caffe2* as a backend for a converted model.
+    Note: This is not the official Python API.
     """  # noqa: E501
 
     @classmethod

requirements.txt

@@ -1,3 +1,2 @@
 numpy >= 1.16.6
-onnx >= 1.2.3
 protobuf