keras/src/layers/reshaping/reshape.py/Reshape/build
class Reshape:    def build(self, input_shape):
        sample_output_shape = operation_utils.compute_reshape_output_shape(
            input_shape[1:], self.target_shape, "target_shape"
        )
        self._resolved_target_shape = tuple(
            -1 if d is None else d for d in sample_output_shape
        )
        self.built = True