keras/src/layers/layer.py/Layer/build
class Layer:    def build(self, input_shape):
        self._check_super_called()
        if utils.is_default(self.build) and might_have_unbuilt_state(self):
            warnings.warn(
                f"`build()` was called on layer '{self.name}', however "
                "the layer does not have a `build()` method implemented "
                "and it looks like it has unbuilt state. This will cause "
                "the layer to be marked as built, despite not being "
                "actually built, which may cause failures down the line. "
                "Make sure to implement a proper `build()` method."
            )
        self.built = True