Fix typo in autodiff formula for addmm (#7932)

diff --git a/torch/csrc/jit/autodiff.cpp b/torch/csrc/jit/autodiff.cpp
index 2658895..3c9077b 100644
--- a/torch/csrc/jit/autodiff.cpp
+++ b/torch/csrc/jit/autodiff.cpp
@@ -95,7 +95,7 @@
             dmat2 = inputs.at(0).t().mm(grads.at(0));
           }
         } else {
-          dmat2 = grads.at(0).mm(inputs.at(1).t());
+          dmat2 = inputs.at(0).t().mm(grads.at(0));
         }
         return {dmat1, dmat2};
       }