chore: import upstream snapshot with attribution

docs/source/guide/message.rst

@@ -0,0 +1,46 @@
+.. _guide-message-passing:
+
+Chapter 2: Message Passing
+==========================
+
+:ref:`(中文版) <guide_cn-message-passing>`
+
+Message Passing Paradigm
+------------------------
+
+Let :math:`x_v\in\mathbb{R}^{d_1}` be the feature for node :math:`v`,
+and :math:`w_{e}\in\mathbb{R}^{d_2}` be the feature for edge
+:math:`({u}, {v})`. The **message passing paradigm** defines the
+following node-wise and edge-wise computation at step :math:`t+1`:
+
+.. math::  \text{Edge-wise: } m_{e}^{(t+1)} = \phi \left( x_v^{(t)}, x_u^{(t)}, w_{e}^{(t)} \right) , ({u}, {v},{e}) \in \mathcal{E}.
+
+.. math::  \text{Node-wise: } x_v^{(t+1)} = \psi \left(x_v^{(t)}, \rho\left(\left\lbrace m_{e}^{(t+1)} : ({u}, {v},{e}) \in \mathcal{E} \right\rbrace \right) \right).
+
+In the above equations, :math:`\phi` is a **message function**
+defined on each edge to generate a message by combining the edge feature
+with the features of its incident nodes; :math:`\psi` is an
+**update function** defined on each node to update the node feature
+by aggregating its incoming messages using the **reduce function**
+:math:`\rho`.
+
+Roadmap
+-------
+
+This chapter introduces DGL's message passing APIs, and how to efficiently use them on both nodes and edges.
+The last section of it explains how to implement message passing on heterogeneous graphs.
+
+* :ref:`guide-message-passing-api`
+* :ref:`guide-message-passing-efficient`
+* :ref:`guide-message-passing-part`
+* :ref:`guide-message-passing-heterograph`
+
+.. toctree::
+    :maxdepth: 1
+    :hidden:
+    :glob:
+
+    message-api
+    message-efficient
+    message-part
+    message-heterograph