ONNX Runtime 源码阅读：子图（sub-graph）应该如何理解？

在 ONNX Runtime 源码（onnxruntime/core/framework/graph_partitioner.cc，函数GraphPartitioner::Partition）中，有这么一段注释：

NOTE: A 'sub-graph' is a subset of nodes within the current Graph instance.
      The control flow nodes have nested Graph instance/s which are also called subgraphs,
      but are completely separate Graph instances and not a subset of nodes within a single Graph instance.

也就是说，ONNX Runtime 中提到的子图有两类：

• 一类是 Graph 实例中节点（nodes）的子集节点，也就是对整图进行切分时所划分的子集；
• 另一类是控制流节点（e.g., if & loop）中提到的子图，但是这些子图是完全独立的 Graph 实例，而非某个 Graph 实例的子集节点