Change backward op infershape

test=develop

paddle/fluid/operators/expand_op.cc

@@ -115,7 +115,7 @@ class ExpandGradOp : public framework::OperatorWithKernel {
     auto out_dims = ctx->GetInputDim(framework::GradVarName("Out"));
 
     size_t start_pos = 0u;
-    if (!ctx->IsRuntime()) {
+    if (!ctx->IsRuntime() && x_dims[0] < 0) {
       PADDLE_ENFORCE_EQ(
           x_dims[0], out_dims[0],
           "The first dimension size of Input(Out@GRAD) should be "