Make it possible to specify multiple config files (#54)

Summary:

Make it possible to specify multiple config files.
Parsing CLI is not a special case anymore, just uses the same config inheritance method.

Test Plan:

Test that this iterpolates in the right order via unit tests

Sample usage, loads the internal config, which references bytelatent/configs/entropy_model.yaml. The precendence order is:

- Default pydantic args
- Included configs, eg `config`
- CLI args

```
python -m bytelatent.print_config config=internal/configs/entropy_model.yaml eval=null

```


Summary:

Test Plan:

bytelatent/config_parser.py

@@ -0,0 +1,73 @@
+import copy
+from typing import Type, TypeVar
+
+import omegaconf
+from omegaconf import DictConfig, OmegaConf
+from pydantic import BaseModel
+
+
+def parse_file_config(path: str) -> DictConfig:
+    file_cfg = OmegaConf.load(path)
+    if not isinstance(file_cfg, DictConfig):
+        raise ValueError(
+            f"File paths must parse to DictConfig, but it was: {type(file_cfg)}"
+        )
+    return file_cfg
+
+
+def recursively_parse_config(cfg: DictConfig) -> list[DictConfig]:
+    if "config" not in cfg:
+        return [cfg]
+
+    ordered_cfgs = []
+    cfg = copy.deepcopy(cfg)
+    config_arg = cfg["config"]
+    del cfg["config"]
+    ordered_cfgs.append(cfg)
+
+    if isinstance(config_arg, str):
+        file_cfg = parse_file_config(config_arg)
+        sub_configs = recursively_parse_config(file_cfg)
+        ordered_cfgs = sub_configs + ordered_cfgs
+    elif isinstance(config_arg, omegaconf.listconfig.ListConfig):
+        sub_configs = []
+        for c in config_arg:
+            if not isinstance(c, str):
+                raise ValueError(
+                    f'If "config" is specified, it must be either a string path or a list of string paths. It was config={config_arg}'
+                )
+            config_to_parse = parse_file_config(c)
+            sub_configs.extend(recursively_parse_config(config_to_parse))
+        ordered_cfgs = sub_configs + ordered_cfgs
+    else:
+        raise ValueError(
+            f'If "config" is specified, it must be either a string path or a list of string paths, it was config={config_arg}'
+        )
+    return ordered_cfgs
+
+
+def parse_args_with_default(
+    *, default_cfg: DictConfig | None = None, cli_args: DictConfig | None = None
+):
+    if cli_args is None:
+        cli_args = OmegaConf.from_cli()
+        assert isinstance(
+            cli_args, DictConfig
+        ), f"CLI Args must be a DictConfig, not {type(cli_args)}"
+    ordered_cfgs = recursively_parse_config(cli_args)
+    if default_cfg is not None:
+        ordered_cfgs.insert(0, default_cfg)
+    cfg = OmegaConf.merge(*ordered_cfgs)
+    return OmegaConf.to_container(cfg, resolve=True, throw_on_missing=True)
+
+
+T = TypeVar("T", bound=BaseModel)
+
+
+def parse_args_to_pydantic_model(
+    args_cls: Type[T], cli_args: DictConfig | None = None
+) -> T:
+    default_cfg = OmegaConf.create(args_cls().model_dump())
+    parsed_cfg = parse_args_with_default(default_cfg=default_cfg, cli_args=cli_args)
+    pydantic_args = args_cls.model_validate(parsed_cfg)
+    return pydantic_args