Load config from checkpoint_dir if weights are not required (#755)

lit_gpt/config.py

@@ -116,6 +116,15 @@ def from_json(cls, path: Union[str, Path], **kwargs: Any) -> Self:
         json_kwargs.update(kwargs)
         return cls(**json_kwargs)
 
+    @classmethod
+    def from_checkpoint(cls, path: Path, **kwargs: Any) -> Self:
+        """Automatically load `lit_config.json` and if it doesn't exist - a matching config from `lit_gpt/config.py`."""
+        if (config_path := path / "lit_config.json").is_file():
+            return cls.from_json(config_path, **kwargs)
+        if (model_name := path.name) in name_to_config:
+            return cls.from_name(model_name, **kwargs)
+        raise FileNotFoundError(f"For {str(path)!r} neither 'lit_config.json' nor matching config exists.")
+
     @property
     def mlp_class(self) -> Type:
         # `self._mlp_class` cannot be the type to keep the config json serializable

scripts/prepare_redpajama.py

@@ -147,7 +147,7 @@ def prepare(
     match: str = "",
 ) -> None:
     """Prepare the "Red Pajama" dataset. We assume tokenizer has been trained."""
-    config = Config.from_json(checkpoint_dir / "lit_config.json")
+    config = Config.from_checkpoint(checkpoint_dir)
 
     prepare_fn = prepare_sample if sample else prepare_full
     prepare_fn(

tests/test_config.py

@@ -93,3 +93,35 @@ def test_nonexisting_name():
 
     with pytest.raises(ValueError, match="not a supported"):
         Config.from_name("foobar")
+
+
+def test_from_checkpoint(tmp_path):
+    from lit_gpt import Config
+
+    # 1. Neither `lit_config.py` nor matching config exists.
+    with pytest.raises(FileNotFoundError, match="neither 'lit_config.json' nor matching config exists"):
+        Config.from_checkpoint(tmp_path / "non_existing_checkpoint")
+
+    # 2. If `lit_config.py` doesn't exists, but there is a matching config in `lit_gpt/config.py`.
+    config = Config.from_checkpoint(tmp_path / "pythia-70m")
+    assert config.name == "pythia-70m"
+    assert config.block_size == 2048
+    assert config.n_layer == 6
+
+    # 3. If only `lit_config.py` exists.
+    config_data = {"name": "pythia-70m", "block_size": 24, "n_layer": 2}
+    with open(tmp_path / "lit_config.json", "w") as file:
+        json.dump(config_data, file)
+    config = Config.from_checkpoint(tmp_path)
+    assert config.name == "pythia-70m"
+    assert config.block_size == 24
+    assert config.n_layer == 2
+
+    # 4. Both `lit_config.py` and a matching config exist, but `lit_config.py` supersedes matching config
+    (tmp_path / "pythia-70m").mkdir()
+    with open(tmp_path / "pythia-70m/lit_config.json", "w") as file:
+        json.dump(config_data, file)
+    config = Config.from_checkpoint(tmp_path / "pythia-70m")
+    assert config.name == "pythia-70m"
+    assert config.block_size == 24
+    assert config.n_layer == 2