Accuracy metric return tuple of length num_gpus on ddp in 0.9.1rc4
code:
        vid_acc = self.accuracy_video(video_labels_hat, video_labels)
        print(len(vid_acc), vid_acc)
        monitor = 0-vid_acc

In 0.9.1rc3, vid acc is a tensor, but in rc4, it changes to a tuple. I want to use -vid_acc as monitor, and I think it should be a tensor.
Using rc4, in macos's cpu mode, it's a tensor. But in linux ddp mode, it's a tuple of length num_gpus.