[ONNX 3] Add ONNX registry tutorial

Author: titaiwangms

_static/img/onnx/custom_addandround_function.png

@@ -8,3 +8,7 @@ ONNX
 2. export_simple_model_to_onnx_tutorial.py
     Export a PyTorch model to ONNX
     https://pytorch.org/tutorials/beginner/onnx/export_simple_model_to_onnx_tutorial.html
+
+3. onnx_registry_tutorial.py
+    Introduction to ONNX
+    https://pytorch.org/tutorials/beginner/onnx/onnx_registry_tutorial.html

_static/img/onnx/custom_addandround_model.png

@@ -32,13 +32,21 @@
 Dependencies
 ------------
 
+PyTorch 2.1.0 or newer is required.
+
 The ONNX exporter depends on extra Python packages:
 
   - `ONNX <https://onnx.ai>`_
   - `ONNX Script <https://onnxscript.ai>`_
 
 They can be installed through `pip <https://pypi.org/project/pip/>`_:
 
+.. note::
+   This tutorial leverages `onnxscript <https://github.com/microsoft/onnxscript#readme>`__
+   to create custom ONNX operators. onnxscript is a Python library that allows users to
+   create custom ONNX operators in Python. It is a prerequisite learning material for
+   this tutorial. Please make sure you have read the onnxscript tutorial before proceeding.
+
 .. code-block:: bash
 
   pip install --upgrade onnx onnxscript