keras/src/backend/common/variables.py/KerasVariable/_validate_shape
class KerasVariable:    def _validate_shape(self, shape):
        shape = standardize_shape(shape)
        if None in shape:
            raise ValueError(
                "Shapes used to initialize variables must be "
                "fully-defined (no `None` dimensions). Received: "
                f"shape={shape} for variable path='{self.path}'"
            )
        return shape