fix split_byref_op infer shape

test=develop

paddle/fluid/operators/distributed_ops/split_byref_op.cc

@@ -31,14 +31,16 @@ class SplitByrefOp : public framework::OperatorWithKernel {
     auto in_dims = ctx->GetInputDim("X");
     auto outs_names = ctx->Outputs("Out");
     size_t num = static_cast<size_t>(ctx->Attrs().Get<int>("num"));
-    std::vector<int> sections = static_cast<std::vector<int>>(
-        ctx->Attrs().Get<std::vector<int>>("sections"));
+    auto sections = ctx->Attrs().Get<std::vector<int>>("sections");
     const size_t outs_number = outs_names.size();
     std::vector<framework::DDim> outs_dims;
     outs_dims.reserve(outs_number);
 
     if (num > 0) {
-      int64_t in_axis_dim = in_dims[0];
+      int64_t in_axis_dim = 0;
+      if (ctx->IsRuntime()) {
+        in_axis_dim = in_dims[0];
+      }
       PADDLE_ENFORCE_EQ(in_axis_dim % num, 0,
                         "tensor split does not result"
                         " in an equal division");