Merge pull request #2238 from f4str/pytorch-object-detector-fix

FIx `PytorchObjectDetector` Loss Gradient Bug

Author: beat-buesser

art/estimators/object_detection/pytorch_object_detector.py

@@ -219,7 +219,10 @@ def _preprocess_and_convert_inputs(
 
             # Set gradients
             if not no_grad:
-                x_tensor.requires_grad = True
+                if x_tensor.is_leaf:
+                    x_tensor.requires_grad = True
+                else:
+                    x_tensor.retain_grad()
 
             # Apply framework-specific preprocessing
             x_preprocessed, y_preprocessed = self._apply_preprocessing(x=x_tensor, y=y_tensor, fit=fit, no_grad=no_grad)