Created by: reyoung

Some method of DeviceContext is not const, such as

class GPUDeviceContext: public DeviceContext {
  cuda_stream_t Stream();
  cunn_handle_t CudnnHandle();
};

The code is here. It because the handlers and stream are initialized lazily, i.e., they will not be initialized only until they are used. So DeviceContext is a mutable object, not const object.

But in Operator::Run, we use DeviceContext like Operator::Run(..., const DeviceContext& context);. So if we want to use cudaStream or cudnnHandle, we must const_cast(context) to drop const qualifier. The code like

auto cudnn_handle = const_cast<DeviceContext&>(context).cudnnHandle();

However, const_cast is very harmful and should be forbidden in our code.