For code such as 'model->model = ov_model' is confusing. We can
just drop the member variable and use cast to get the subclass.
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
Reviewed-by: Wenbin Chen <wenbin.chen@intel.com>
Reviewed-by: Guo Yejun <yejun.guo@intel.com>
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