docs(mgb/opr): improve batch_norm err msg in training mode

GitOrigin-RevId: 4c43bc8b0b08605238b8eaa0a1ce67d2dae5e348

src/opr/impl/dnn/batch_norm.cpp

@@ -50,8 +50,9 @@ BatchNormForward::BatchNormForward(VarNode *x,
             mgb_throw_if(!(dest_opr->same_type<SharedDeviceTensor>() ||
                     dest_opr->same_type<VolatileSharedDeviceTensor>()),
                     GraphError,
-                    "mean and variance in BatchNorm must be SharedDeviceTensor "
-                    "or VolatileSharedDeviceTensor; got %s{%s} actually",
+                    "mean and variance in training mode BatchNorm must be",
+                    "SharedDeviceTensor or VolatileSharedDeviceTensor;",
+                    "got %s{%s} actually",
                     dest_opr->cname(), dest_opr->dyn_typeinfo()->name);
         };
         check_dest(mean);