Loads and parses a demonstration file.
    :param file_path: Location of demonstration file (.demo).
    :return: BrainParameter and list of BrainInfos containing demonstration data.

def load_demonstration(file_path):
    """
    Loads and parses a demonstration file.
    :param file_path: Location of demonstration file (.demo).
    :return: BrainParameter and list of BrainInfos containing demonstration data.
    """

    # First 32 bytes of file dedicated to meta-data.
    INITIAL_POS = 33

    if not os.path.isfile(file_path):
        raise FileNotFoundError("The demonstration file {} does not exist.".format(file_path))
    file_extension = pathlib.Path(file_path).suffix
    if file_extension != '.demo':
        raise ValueError("The file is not a '.demo' file. Please provide a file with the "
                         "correct extension.")

    brain_params = None
    brain_infos = []
    data = open(file_path, "rb").read()
    next_pos, pos, obs_decoded = 0, 0, 0
    total_expected = 0
    while pos < len(data):
        next_pos, pos = _DecodeVarint32(data, pos)
        if obs_decoded == 0:
            meta_data_proto = DemonstrationMetaProto()
            meta_data_proto.ParseFromString(data[pos:pos + next_pos])
            total_expected = meta_data_proto.number_steps
            pos = INITIAL_POS
        if obs_decoded == 1:
            brain_param_proto = BrainParametersProto()
            brain_param_proto.ParseFromString(data[pos:pos + next_pos])
            brain_params = BrainParameters.from_proto(brain_param_proto)
            pos += next_pos
        if obs_decoded > 1:
            agent_info = AgentInfoProto()
            agent_info.ParseFromString(data[pos:pos + next_pos])
            brain_info = BrainInfo.from_agent_proto([agent_info], brain_params)
            brain_infos.append(brain_info)
            if len(brain_infos) == total_expected:
                break
            pos += next_pos
        obs_decoded += 1
    return brain_params, brain_infos, total_expected