Update adaptive_mean_pooling_impl.hpp

Fixed Serialization issue.

src/mlpack/methods/ann/layer/adaptive_mean_pooling_impl.hpp

@@ -146,6 +146,8 @@ void AdaptiveMeanPoolingType<MatType>::serialize(
   ar(cereal::base_class<Layer<MatType>>(this));
 
   ar(CEREAL_NVP(poolingLayer));
+  ar(CEREAL_NVP(outputWidth));
+  ar(CEREAL_NVP(outputHeight));
 }
 
 } // namespace ann