Parse draccus subclass overrides when using --policy.path (#1501)

* Parse draccus subclass overrides when using --policy.path

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Simon Alibert <75076266+aliberts@users.noreply.github.com>

src/lerobot/configs/policies.py

@@ -12,8 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import abc
+import json
 import logging
 import os
+import tempfile
 from dataclasses import dataclass, field
 from pathlib import Path
 from typing import Type, TypeVar
@@ -183,8 +185,22 @@ class PreTrainedConfig(draccus.ChoiceRegistry, HubMixin, abc.ABC):
                     f"{CONFIG_NAME} not found on the HuggingFace Hub in {model_id}"
                 ) from e
 
-        # HACK: this is very ugly, ideally we'd like to be able to do that natively with draccus
+        # HACK: Parse the original config to get the config subclass, so that we can
+        # apply cli overrides.
+        # This is very ugly, ideally we'd like to be able to do that natively with draccus
         # something like --policy.path (in addition to --policy.type)
-        cli_overrides = policy_kwargs.pop("cli_overrides", [])
         with draccus.config_type("json"):
-            return draccus.parse(cls, config_file, args=cli_overrides)
+            orig_config = draccus.parse(cls, config_file, args=[])
+
+        with open(config_file) as f:
+            config = json.load(f)
+
+        config.pop("type")
+        with tempfile.NamedTemporaryFile("w+") as f:
+            json.dump(config, f)
+            config_file = f.name
+            f.flush()
+
+            cli_overrides = policy_kwargs.pop("cli_overrides", [])
+            with draccus.config_type("json"):
+                return draccus.parse(orig_config.__class__, config_file, args=cli_overrides)