add support for Apple hardware using MPS acceleration

ldm/dream/devices.py

@@ -0,0 +1,11 @@
+import torch
+
+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():
+        return 'mps'
+    return 'cpu'
+
+