[recipe] fix: init self.model_config in fsdp worker of one-step-off policy (#3556)

### What does this PR do?

Due to updated in the main package, the rollout worker calls
`self.model_config` during `generate_sequences`
(https://github.com/volcengine/verl/blob/d33c85e2c779da1203e54275645b7b30f7fe3ce1/verl/workers/fsdp_workers.py#L869)
which hasn't been initialized in current one-step-off recipe. This will
through out runtime errors.

Similar code in the default fsdp worker:
https://github.com/volcengine/verl/blob/d33c85e2c779da1203e54275645b7b30f7fe3ce1/verl/workers/fsdp_workers.py#L563

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Author: zlwang-cs

recipe/one_step_off_policy/fsdp_workers.py

@@ -220,6 +220,7 @@ def init_model(self):
 
         rollout_config: RolloutConfig = omega_conf_to_dataclass(self.config.rollout)
         model_config: HFModelConfig = omega_conf_to_dataclass(self.config.model, dataclass_type=HFModelConfig)
+        self.model_config = model_config
 
         log_gpu_memory_usage(f"Before building {rollout_name} rollout", logger=logger)
         rollout = get_rollout_class(rollout_config.name, rollout_config.mode)(