Pinning memory issue

[Li-Huai (Allan) Lin]

Hi,

I'm currently using ckip-transformers-ws as a preprocessing tool in my project, and I noticed that the DataLoader's pin_memory flag was hard-coded True in util.py.

As pinning memory is incompatible with multiprocessing (or multiple workers) [1], when users leverage ckip-transformers in their collate_fn of DataLoader with multiple workers, a CUDA error will occur as shown in [1], even if only using CPU for inference.

Therefore, I think it would be better that:

1. Pin memory only when the device is GPU.
2. Add an option to decide whether or not to enable memory pinning.

Regards.

[1] https://discuss.pytorch.org/t/pin-memory-vs-sending-direct-to-gpu-from-dataset/33891/2

[Mu Yang]

Implemented in 0.2.7.