Fix TensorExtensions.StdDev (dotnet#110392)

* Fix TensorExtensions.StdDev

* Add constraint to ref

* Revert "Add constraint to ref"

This reverts commit f740f50.

* Revert "Fix TensorExtensions.StdDev"

This reverts commit c212984.

* Use pow method

* Use existing variable

src/libraries/System.Numerics.Tensors/src/System/Numerics/Tensors/netcore/TensorExtensions.cs

@@ -3521,7 +3521,17 @@ public static T StdDev<T>(in ReadOnlyTensorSpan<T> x)
             TensorPrimitives.Abs(output, output);
             TensorPrimitives.Pow((ReadOnlySpan<T>)output, T.CreateChecked(2), output);
             T sum = TensorPrimitives.Sum((ReadOnlySpan<T>)output);
-            return T.CreateChecked(sum / T.CreateChecked(x._shape._memoryLength));
+            T variance = sum / T.CreateChecked(x._shape._memoryLength);
+
+            if (typeof(T) == typeof(float))
+            {
+                return T.CreateChecked(MathF.Sqrt(float.CreateChecked(variance)));
+            }
+            if (typeof(T) == typeof(double))
+            {
+                return T.CreateChecked(Math.Sqrt(double.CreateChecked(variance)));
+            }
+            return T.Pow(variance, T.CreateChecked(0.5));
         }
         #endregion
 

src/libraries/System.Numerics.Tensors/tests/TensorTests.cs

@@ -1113,7 +1113,7 @@ public static float StdDev(float[] values)
             {
                 sum += MathF.Pow(values[i] - mean, 2);
             }
-            return sum / values.Length;
+            return MathF.Sqrt(sum / values.Length);
         }
 
         [Fact]