Build a Trial instance from a json string.

def from_json(cls, json_info):
        """Build a Trial instance from a json string."""
        if json_info is None:
            return None
        return TrialRecord(
            trial_id=json_info["trial_id"],
            job_id=json_info["job_id"],
            trial_status=json_info["status"],
            start_time=json_info["start_time"],
            params=json_info["params"])