Use TF-native NMS when in TF backend (keras-team#1931)

* Use TF-native NMS when in TF backend

* I promise I have used a computer before

keras_cv/layers/object_detection/non_max_suppression.py

@@ -88,60 +88,53 @@ def call(
 
         confidence_prediction = ops.max(class_prediction, axis=-1)
 
-        # TODO(tirthasheshpatel): Use backend-specific op where available
-        if multi_backend():
+        if not multi_backend() or keras.backend.backend() == "tensorflow":
+            idx, valid_det = tf.image.non_max_suppression_padded(
+                box_prediction,
+                confidence_prediction,
+                max_output_size=self.max_detections,
+                iou_threshold=self.iou_threshold,
+                score_threshold=self.confidence_threshold,
+                pad_to_max_output_size=True,
+                sorted_input=False,
+            )
+        elif keras.backend.backend() == "torch":
             # Since TorchVision has a nice efficient NMS op, we might as well
             # use it!
-            if keras.backend.backend() == "torch":
-                import torchvision
-
-                batch_size = box_prediction.shape[0]
-                idx = ops.zeros((batch_size, self.max_detections))
-                valid_det = ops.zeros((batch_size), "int32")
-
-                for batch_idx in range(batch_size):
-                    conf_mask = (
-                        confidence_prediction[batch_idx]
-                        > self.confidence_threshold
-                    )
-                    conf_mask_idx = ops.squeeze(ops.nonzero(conf_mask), axis=0)
-                    conf_i = confidence_prediction[batch_idx][conf_mask]
-                    box_i = box_prediction[batch_idx][conf_mask]
-
-                    idx_i = torchvision.ops.nms(
-                        box_i, conf_i, iou_threshold=self.iou_threshold
-                    )
-
-                    idx_i = conf_mask_idx[idx_i]
-
-                    num_boxes = idx_i.shape[0]
-                    if num_boxes >= self.max_detections:
-                        idx_i = idx_i[: self.max_detections]
-                        num_boxes = self.max_detections
-
-                    valid_det[batch_idx] = ops.cast(ops.size(idx_i), "int32")
-                    idx[batch_idx, :num_boxes] = idx_i
-
-            else:
-                idx, valid_det = non_max_suppression(
-                    box_prediction,
-                    confidence_prediction,
-                    max_output_size=self.max_detections,
-                    iou_threshold=self.iou_threshold,
-                    score_threshold=self.confidence_threshold,
+            import torchvision
+
+            batch_size = box_prediction.shape[0]
+            idx = ops.zeros((batch_size, self.max_detections))
+            valid_det = ops.zeros((batch_size), "int32")
+
+            for batch_idx in range(batch_size):
+                conf_mask = (
+                    confidence_prediction[batch_idx] > self.confidence_threshold
+                )
+                conf_mask_idx = ops.squeeze(ops.nonzero(conf_mask), axis=0)
+                conf_i = confidence_prediction[batch_idx][conf_mask]
+                box_i = box_prediction[batch_idx][conf_mask]
+
+                idx_i = torchvision.ops.nms(
+                    box_i, conf_i, iou_threshold=self.iou_threshold
                 )
+
+                idx_i = conf_mask_idx[idx_i]
+
+                num_boxes = idx_i.shape[0]
+                if num_boxes >= self.max_detections:
+                    idx_i = idx_i[: self.max_detections]
+                    num_boxes = self.max_detections
+
+                valid_det[batch_idx] = ops.cast(ops.size(idx_i), "int32")
+                idx[batch_idx, :num_boxes] = idx_i
         else:
-            # For non-multibackend, our NMS fails during graph tracing due to
-            # the lack of a defined batch size, so we just fall back to the
-            # original implementation that this is ported from.
-            idx, valid_det = tf.image.non_max_suppression_padded(
+            idx, valid_det = non_max_suppression(
                 box_prediction,
                 confidence_prediction,
                 max_output_size=self.max_detections,
                 iou_threshold=self.iou_threshold,
                 score_threshold=self.confidence_threshold,
-                pad_to_max_output_size=True,
-                sorted_input=False,
             )
 
         box_prediction = ops.take_along_axis(