"""Default implementation for getting meta information about the temporary data collected between get_batch calls.

This function calculates and returns a dictionary containing meta information about the temporary data, including the total number of samples, the number of groups, the average group size, and the average reward across all samples.

Args:
    temp_data (Dict[str, List[Dict[str, Any]]]): A dictionary where each key is a group identifier and each value is a list of samples. Each sample is a dictionary that must contain a 'reward' key.

Returns:
    Dict[str, Any]: A dictionary with the following keys:
        - 'total_samples' (int): The total number of samples across all groups.
        - 'num_groups' (int): The number of groups in the temporary data.
        - 'avg_group_size' (float): The average number of samples per group.
        - 'avg_reward' (float): The average reward across all samples.

If the input `temp_data` is empty, the function returns a dictionary with all values set to 0.
"""