Save metrics in a json file

def save_data(self):
        """ Save metrics in a json file """
        if len(self.data.keys()) == 0:
            return

        # Sort everything by episode.
        assert 'episode' in self.data
        sorted_indexes = np.argsort(self.data['episode'])
        sorted_data = {}
        for key, values in self.data.items():
            assert len(self.data[key]) == len(sorted_indexes)
            # We convert to np.array() and then to list to convert from np datatypes to native datatypes.
            # This is necessary because json.dump cannot handle np.float32, for example.
            sorted_data[key] = np.array([self.data[key][idx] for idx in sorted_indexes]).tolist()

        # Overwrite already open file. We can simply seek to the beginning since the file will
        # grow strictly monotonously.
        with open(self.filepath, 'w') as f:
            json.dump(sorted_data, f)