Fix GPU clustering

A variable which did not support GPU operations was errantly on GPU when
cuda=True. Move to CPU.

vamb/cluster.py

@@ -349,7 +349,7 @@ def _smaller_indices(tensor, kept_mask, threshold, cuda):
 
     # If it's on GPU, we remove the already clustered points at this step.
     if cuda:
-        return _torch.nonzero((tensor <= threshold) & kept_mask).flatten()
+        return _torch.nonzero((tensor <= threshold) & kept_mask).flatten().cpu()
     else:
         arr = tensor.numpy()
         indices = (arr <= threshold).nonzero()[0]