Polish code

test=develop

Author: velconia

paddle/fluid/operators/expand_op.cc

@@ -117,7 +117,7 @@ class ExpandGradOp : public framework::OperatorWithKernel {
     size_t start_pos = 0u;
     if (!ctx->IsRuntime()) {
       PADDLE_ENFORCE_EQ(
-          x_dims[i], out_dims[i],
+          x_dims[0], out_dims[0],
           "The first dimension size of Input(Out@GRAD) should be "
           "equal to the crroresponding dimension size of Input(X)");
       start_pos = 1u;