Validates export_formats.

        Raises:
            ValueError if the format is unknown.

def validate(export_formats):
        """Validates export_formats.

        Raises:
            ValueError if the format is unknown.
        """
        for i in range(len(export_formats)):
            export_formats[i] = export_formats[i].strip().lower()
            if export_formats[i] not in [
                    ExportFormat.CHECKPOINT, ExportFormat.MODEL
            ]:
                raise TuneError("Unsupported export format: " +
                                export_formats[i])