Fix overestimation in MachineLearning.NQubitsRequired

MachineLearning/src/Structure.qs

@@ -20,17 +20,14 @@ namespace Microsoft.Quantum.MachineLearning {
     /// may be applied.
     function NQubitsRequired(model : SequentialModel)
     : Int {
-        mutable nQubitsRequired = 0;
+        mutable lastQubitIndex = -1; // Need to return 0 if there are no gates
         for gate in model::Structure {
-            set nQubitsRequired = 1 + Fold(
-                MaxI, 0,
-                gate::ControlIndices + [
-                    gate::TargetIndex,
-                    nQubitsRequired
-                ]
+            set lastQubitIndex = Fold(
+                MaxI, lastQubitIndex,
+                gate::ControlIndices + [gate::TargetIndex]
             );
         }
-        return nQubitsRequired;
+        return lastQubitIndex + 1;
     }
 
     /// # Summary

MachineLearning/tests/StructureTests.qs

@@ -12,7 +12,8 @@ namespace Microsoft.Quantum.MachineLearning.Tests {
     function NQubitsRequiredFact() : Unit {
         let model = Default<ML.SequentialModel>()
             w/ Structure <- [
-                ML.ControlledRotation((3, [7, 9]), PauliX, 0)
+                ML.ControlledRotation((3, [7, 9]), PauliX, 0),
+                ML.ControlledRotation((8, new Int[0]), PauliY, 1)
             ];
         let actual = ML.NQubitsRequired(model);
         EqualityFactI(actual, 10, "Wrong output from NQubitsRequired.");