fix pytorch object detector gradients bug

Signed-off-by: Farhan Ahmed <[email protected]>

Author: f4str

art/estimators/object_detection/pytorch_object_detector.py

@@ -270,6 +270,12 @@ def _get_losses(
         x_preprocessed = x_preprocessed.to(self.device)
         y_preprocessed = [{k: v.to(self.device) for k, v in y_i.items()} for y_i in y_preprocessed]
 
+        # Set gradients again after inputs are moved to another device
+        if x_preprocessed.is_leaf:
+            x_preprocessed.requires_grad = True
+        else:
+            x_preprocessed.retain_grad()
+
         loss_components = self._model(x_preprocessed, y_preprocessed)
 
         return loss_components, x_preprocessed