Fixed up dimensions to create known sized vectors

src/Microsoft.ML.OnnxTransformer/OnnxTypeParser.cs

@@ -201,6 +201,16 @@ public static IEnumerable<int> GetTensorDims(Microsoft.ML.Model.OnnxConverter.On
                 var dimValue = GetDimValue(d);
                 dims.Add(dimValue);
             }
+
+            // In ONNX, the first dimension refers to the batch size. If that is set to -1, it means OnnxRuntime can do inferencing in batches on
+            // multiple rows at once. In ML.NET, a vector is considered to be of known size if the dimensions are all greater than zero
+            // Leaving the batch size at -1 causes all Onnx vectors to be considered to be of unknown size. Therefore, if the first dimension is -1,
+            // we need to fix up the shape. But GetDimValue above converts any dimension < 0 to be 0. We need that behavior for dimensions other than
+            // the first dimension. So we check only the first dimension here and fix it up. (The '<=' comparison below is there to make sure that
+            // this holds even if the behavior of GetDimValue changes).
+            if ((dims.Count > 0) && (dims[0] <= 0))
+                dims[0] = 1;
+
             return dims;
         }