message: Add back optional input to the convolution.
diff: @@ -98,7 +98,7 @@ The convolution operator consumes an input tensor and {filter_desc}, and
  computes the output.)DOC";
  ReplaceAll(doc, "{filter_desc}", filter_desc);
  schema.SetDoc(doc);
  - schema.NumInputs(2);
  + schema.NumInputs(2, 3);
  schema.NumOutputs(1);
  schema.Input(0,
  "X",
  