Correct get_device Warnings (#698)

* Ensure warning only thrown if user requests difference divice than is given

* Make requested changes

Author: mauicv

alibi_detect/utils/pytorch/misc.py

@@ -89,5 +89,6 @@ def get_device(device: Optional[Union[str, torch.device]] = None) -> torch.devic
                 logger.warning('No GPU detected, fall back on CPU.')
         else:
             torch_device = torch.device('cpu')
-            logger.warning('Requested device not recognised, fall back on CPU.')
+            if device.lower() != 'cpu':
+                logger.warning('Requested device not recognised, fall back on CPU.')
     return torch_device