Fix image.shape type (keras-team#2305)

Fixed image.shape issue for tensorflow backend

keras_cv/layers/object_detection/anchor_generator.py

@@ -172,7 +172,7 @@ def __call__(self, image=None, image_shape=None):
                     "Expected `image` to be a Tensor of rank 3. Got "
                     f"image.shape.rank={len(image.shape)}"
                 )
-            image_shape = image.shape
+            image_shape = tuple(image.shape)
 
         results = {}
         for key, generator in self.anchor_generators.items():