Fix: stride and coord on different devices

[Gofinge]

Fix the bug min_coords and tensor_stride on different devices and raise an error on line 414. A better solution is to assign value to self.device even if it is None in self.init? Then we can make all new tensors can be generated on the specific device directly, instead of .to().

[Gofinge]

More specific descriptions of this bug: Assume self.C and self.F are on 'cuda' -> 'min_coord = min(self.C) is on 'cuda' -> tensor_stride = torch.IntTensor() is on 'cpu' -> Boom.