Fix expand grad op infer shape

test=develop

Author: velconia

paddle/fluid/operators/expand_op.cc

@@ -114,7 +114,12 @@ class ExpandGradOp : public framework::OperatorWithKernel {
         ctx->Attrs().Get<std::vector<int>>("expand_times");
     auto out_dims = ctx->GetInputDim(framework::GradVarName("Out"));
 
-    for (size_t i = 0; i < expand_times.size(); ++i) {
+    size_t start_pos = 0u;
+    if (!ctx->IsRuntime()) {
+      start_pos = 1u;
+    }
+
+    for (size_t i = start_pos; i < expand_times.size(); ++i) {
       PADDLE_ENFORCE_EQ(x_dims[i] * expand_times[i], out_dims[i],
                         "Each dimension size of Input(Out@GRAD) should be "
                         "equal to multiplication of crroresponding dimension "