Make ObsSpec.Shape and DimProps internal, add get props

com.unity.ml-agents/Runtime/Sensors/ObservationSpec.cs

     /// </summary>
     public struct ObservationSpec
     {
+        internal InplaceArray<int> m_Shape;
+
-        public InplaceArray<int> Shape;
+        public InplaceArray<int> Shape
+        {
+            get => m_Shape;
+        }
+
+        internal InplaceArray<DimensionProperty> m_DimensionProperties;
 
         /// <summary>
         /// The properties of each dimensions of the observation.
-        /// It is generally recommended to not modify this from the default values,
+        /// It is generally recommended to use default values provided by helper functions,
-        public InplaceArray<DimensionProperty> DimensionProperties;
+        public InplaceArray<DimensionProperty> DimensionProperties
+        {
+            get => m_DimensionProperties;
+        }
 
         /// <summary>
         /// The type of the observation, e.g. whether they are generic or
             {
                 throw new UnityAgentsException("shape and dimensionProperties must have the same length.");
             }
-            Shape = shape;
-            DimensionProperties = dimensionProperties;
+            m_Shape = shape;
+            m_DimensionProperties = dimensionProperties;
             ObservationType = observationType;
         }
     }

com.unity.ml-agents/Runtime/Sensors/StackingSensor.cs

             m_Name = $"StackingSensor_size{numStackedObservations}_{wrapped.GetName()}";
 
             m_WrappedSpec = wrapped.GetObservationSpec();
-            m_ObservationSpec = m_WrappedSpec;
+            // Set up the cached observation spec for the StackingSensor
+            var newShape = m_WrappedSpec.Shape;
-            m_ObservationSpec.Shape[m_ObservationSpec.NumDimensions - 1] *= numStackedObservations;
+            newShape[newShape.Length - 1] *= numStackedObservations;
+            m_ObservationSpec = new ObservationSpec(
+                newShape, m_WrappedSpec.DimensionProperties, m_WrappedSpec.ObservationType
+            );
 
             // Initialize uncompressed buffer anyway in case python trainer does not
             // support the compression mapping and has to fall back to uncompressed obs.