Fix torchtext data to gpu (#4785)

Co-authored-by: chaton <thomas@grid.ai>

pytorch_lightning/utilities/apply_func.py

@@ -113,7 +113,9 @@ def move_data_to_device(batch: Any, device: torch.device):
 
             # Shallow copy because each Batch has a reference to Dataset which contains all examples
             device_data = copy(data)
-            for field in data.fields:
+            for field, field_value in data.dataset.fields.items():
+                if field_value is None:
+                    continue
                 device_field = move_data_to_device(getattr(data, field), device)
                 setattr(device_data, field, device_field)
             return device_data