Write output_dir in config.yaml in plain path

The value was serialized as a CustomPath object under the old package name, so loading the config raised a YAML ConstructorError.

resplat_init/config.yaml

@@ -451,9 +451,7 @@ meta_trainer:
     weight_decay: 0.01
     warm_up_ratio: 0.01
     adamw_8bit: false
-output_dir: !!python/object/apply:optgs.misc.io.CustomPath
-- checkpoints
-- resplat_init
+output_dir: checkpoints/resplat_init
 use_plugins: false
 log_slurm_id: true
 version: 2.5