keras/src/utils/backend_utils.py/DynamicBackend/set_backend
class DynamicBackend:    def set_backend(self, backend):
        if backend not in ("tensorflow", "jax", "torch", "numpy"):
            raise ValueError(
                "Available backends are ('tensorflow', 'jax', 'torch' and "
                f"'numpy'). Received: backend={backend}"
            )
        self._backend = backend