infer feed operator output variable shape with dims attribute

paddle/operators/feed_op.cc

@@ -32,8 +32,12 @@ class FeedOp : public framework::OperatorWithKernel {
         g_feed_variable->Get<std::vector<framework::Tensor>>();
 
     PADDLE_ENFORCE_GT(tensors.size(), static_cast<size_t>(col));
-    auto in_dim = tensors[col].dims();
-    ctx->SetOutputDim("Out", in_dim);
+
+    auto& shape = ctx->Attrs().Get<std::vector<int>>("dims");
+    std::vector<int64_t> shape_int64(shape.size(), 0);
+    std::transform(shape.begin(), shape.end(), shape_int64.begin(),
+                   [](int a) { return static_cast<int64_t>(a); });
+    ctx->SetOutputDim("Out", framework::make_ddim(shape_int64));
     // TODO(qijun): need to handle LodTensor later
   }