add comments about matrix dimensions

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

@@ -41,6 +41,11 @@ void Lookup<InputDataType, OutputDataType>::Forward(
 
   for (size_t i = 0; i < batchSize; ++i)
   {
+    //! ith column of output is a vectorized form of a matrix of shape
+    //! (seqLength, embeddingSize) selected as a combination of rows from the
+    //! weights. It is done to ensure that the feature dimension remains beside
+    //! the batch dimension when vectorized form is unpacked for some other
+    //! layer.
     output.col(i) = arma::vectorise(weights.rows(
         arma::conv_to<arma::uvec>::from(input.col(i)) - 1));
   }