adding InputTensor class for symmetry with OutputTensor

PiperOrigin-RevId: 168250085

tensorflow/core/graph/graph.h

@@ -257,6 +257,15 @@ class Node {
   TF_DISALLOW_COPY_AND_ASSIGN(Node);
 };
 
+// Represents an input of a node, i.e., the `index`-th input to `node`.
+struct InputTensor {
+  const Node* node;
+  int index;
+
+  InputTensor(const Node* n, int i) : node(n), index(i) {}
+  InputTensor() : node(nullptr), index(0) {}
+};
+
 // Represents an output of a node, i.e., the `index`-th output of `node`. Note
 // that a single `OutputTensor` can correspond to multiple `Edge`s if the output
 // is consumed by multiple destination nodes.
@@ -268,8 +277,6 @@ struct OutputTensor {
   OutputTensor() : node(nullptr), index(0) {}
 };
 
-// TODO(skyewm): add InputTensor if/when necessary
-
 class Edge {
  public:
   Node* src() const { return src_; }