Revert the matrix autos from #801.

stan/math/rev/mat/fun/multiply.hpp

@@ -98,11 +98,13 @@ class multiply_mat_vari : public vari {
     using Eigen::Map;
     using Eigen::MatrixXd;
     MatrixXd adjAB(A_rows_, B_cols_);
+    MatrixXd adjA(A_rows_, A_cols_);
+    MatrixXd adjB(A_cols_, B_cols_);
 
     for (size_type i = 0; i < adjAB.size(); ++i)
       adjAB(i) = variRefAB_[i]->adj_;
-    auto adjA = adjAB * Map<MatrixXd>(Bd_, A_cols_, B_cols_).transpose();
-    auto adjB = Map<MatrixXd>(Ad_, A_rows_, A_cols_).transpose() * adjAB;
+    adjA = adjAB * Map<MatrixXd>(Bd_, A_cols_, B_cols_).transpose();
+    adjB = Map<MatrixXd>(Ad_, A_rows_, A_cols_).transpose() * adjAB;
     for (size_type i = 0; i < A_size_; ++i)
       variRefA_[i]->adj_ += adjA(i);
     for (size_type i = 0; i < B_size_; ++i)
@@ -266,10 +268,11 @@ class multiply_mat_vari<double, Ra, Ca, Tb, Cb> : public vari {
     using Eigen::Map;
     using Eigen::MatrixXd;
     MatrixXd adjAB(A_rows_, B_cols_);
+    MatrixXd adjB(A_cols_, B_cols_);
 
     for (size_type i = 0; i < adjAB.size(); ++i)
       adjAB(i) = variRefAB_[i]->adj_;
-    auto adjB = Map<MatrixXd>(Ad_, A_rows_, A_cols_).transpose() * adjAB;
+    adjB = Map<MatrixXd>(Ad_, A_rows_, A_cols_).transpose() * adjAB;
     for (size_type i = 0; i < B_size_; ++i)
       variRefB_[i]->adj_ += adjB(i);
   }
@@ -426,10 +429,11 @@ class multiply_mat_vari<Ta, Ra, Ca, double, Cb> : public vari {
     using Eigen::Map;
     using Eigen::MatrixXd;
     MatrixXd adjAB(A_rows_, B_cols_);
+    MatrixXd adjA(A_rows_, A_cols_);
 
     for (size_type i = 0; i < adjAB.size(); ++i)
       adjAB(i) = variRefAB_[i]->adj_;
-    auto adjA = adjAB * Map<MatrixXd>(Bd_, A_cols_, B_cols_).transpose();
+    adjA = adjAB * Map<MatrixXd>(Bd_, A_cols_, B_cols_).transpose();
     for (size_type i = 0; i < A_size_; ++i)
       variRefA_[i]->adj_ += adjA(i);
   }