Fix error when default_scope is None

[Y-T-G]

Motivation

Some models don't have any default scope, which means

        if not DefaultScope.check_instance_created(self.experiment_name):
            self.scope = DefaultScope.get_instance(
                self.experiment_name,
                scope_name=self.model_cfg.get('default_scope'))
                )

returns None which causes an error because scope_name should be string.

Modification

Convert to string while retrieving value for scope_name

[CLAassistant]

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[RunningLeon]

@Y-T-G hi, thanks for your PR? could you pose your env and error log when returns None?

[RunningLeon]

LGTM