message: set memory_size for virtual_cluster, to make user specified per_process_gpu_memory_fraction take effect
diff: @@ -45,6 +45,8 @@ VirtualCluster::VirtualCluster(const DeviceSet* device_set)
  for (const auto& device : device_set_->devices()) {
  DeviceProperties props = GetDeviceInfo(device->parsed_name());
  if (props.type() == "UNKNOWN") continue;
  + auto attrs = device->attributes();
  + props.set_memory_size(attrs.memory_limit());
  devices_[device->name()] = props;
  }
  }
  