Fix visual observation tensor indexing for Unity inference. (#6239)

This change corrects the tensor indexing calculation in TensorExtensions.Index()
to properly support CHW (channels-height-width) format used by both Unity's
observation writers and ONNX models during inference.

Author: pglagol

com.unity.ml-agents/Runtime/Inference/TensorExtensions.cs

@@ -57,9 +57,9 @@ public static int Index(this TensorShape shape, int n, int c, int h, int w)
         {
             int index =
                 n * shape.Height() * shape.Width() * shape.Channels() +
-                h * shape.Width() * shape.Channels() +
-                w * shape.Channels() +
-                c;
+                c * shape.Height() * shape.Width() +
+                h * shape.Width() +
+                w;
             return index;
         }
     }