Load a config file and merge it into the default options.

def cfg_from_file(self, yaml_filename, config_dict):
        """Load a config file and merge it into the default options."""
        import yaml
        from easydict import EasyDict as edict
        with open(yaml_filename, 'r') as f:
            yaml_cfg = edict(yaml.load(f))

        return self._merge_a_into_b(yaml_cfg, config_dict)