Fixing conversion of box formats on the original boxes (#2213)

* chore: copy the original boxes and apply convert format

* chore: renaming variable

keras_cv/bounding_box/converters.py

@@ -370,15 +370,16 @@ def convert_format(
             `"float32"`.
     """
     if isinstance(boxes, dict):
-        boxes["boxes"] = convert_format(
+        converted_boxes = boxes.copy()
+        converted_boxes["boxes"] = convert_format(
             boxes["boxes"],
             source=source,
             target=target,
             images=images,
             image_shape=image_shape,
             dtype=dtype,
         )
-        return boxes
+        return converted_boxes
 
     if boxes.shape[-1] is not None and boxes.shape[-1] != 4:
         raise ValueError(