Compatibility with PyTorch DataLoader

Hi,

I tried to combine `decord`'s GPU decoding with PyTorch's `DataLoader`. The code snippet looks like the following:
```
class VideoDataSet(torch.utils.data.Dataset):
...
def __getitem__(self, index):
decord.bridge.set_bridge('torch')
vr = decord.VideoReader(self.video_list[index], ctx=decord.gpu(int(os.getenv('LOCAL_RANK'))))
frames = vr.get_batch(indices)
return frames

if __name__ == '__main__':
torch.multiprocessing.set_start_method("spawn") # avoid CUDA initialization error
dataset = VideoDataSet("kinetics/K700/video_raw", "kinetics/K700/k700_train_video.txt")
loader = torch.utils.data.DataLoader(dataset=dataset, batch_size=16, num_workers=16)
```

The following information shows up multiple times in the training log but no input data is consumed.
Reducing `num_workers` to 1 or even 0 doesn't change the situation.
```
[15:34:05] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:36: Using device: Tesla V100-SXM2-32GB
[15:34:07] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:56: Kernel module version 418.116, so using our own stream.
[15:34:12] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:36: Using device: Tesla V100-SXM2-32GB
[15:34:12] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:56: Kernel module version 418.116, so using our own stream.
[15:34:13] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:36: Using device: Tesla V100-SXM2-32GB
[15:34:13] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:56: Kernel module version 418.116, so using our own stream.
[15:34:13] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:36: Using device: Tesla V100-SXM2-32GB
[15:34:13] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:56: Kernel module version 418.116, so using our own stream.
[15:34:13] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:36: Using device: Tesla V100-SXM2-32GB
[15:34:13] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:56: Kernel module version 418.116, so using our own stream.
[15:34:13] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:36: Using device: Tesla V100-SXM2-32GB
[15:34:13] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:56: Kernel module version 418.116, so using our own stream.
[15:34:14] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:36: Using device: Tesla V100-SXM2-32GB
[15:34:14] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:56: Kernel module version 418.116, so using our own stream.
[15:34:14] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:36: Using device: Tesla V100-SXM2-32GB
[15:34:14] /decord/src/video/nvcodec/cuda_threaded_decoder.cc:56: Kernel module version 418.116, so using our own stream.
```

`decord`'s GPU decoding, when directly used without `Dataset` or `DataLoader`, works well in the same environment.