check if torch.backends has mps before calling it (#245)

Co-authored-by: James Reynolds <[email protected]>

Author: magnusviri

ldm/dream/devices.py

@@ -4,7 +4,7 @@ def choose_torch_device() -> str:
     '''Convenience routine for guessing which GPU device to run model on'''
     if torch.cuda.is_available():
         return 'cuda'
-    if torch.backends.mps.is_available():
+    if hasattr(torch.backends, 'mps') and torch.backends.mps.is_available():
         return 'mps'
     return 'cpu'