The work was completed during the visit of the first author to the Indian Statistical Institute, Chennai Centre and this author thanks the institute for the support and the hospitality. The research of the first author was supported by the NSFs of China (No. 11571049 ), the Construct Program of the Key Discipline in Hunan Province , the Science and Technology Plan Project of Hunan Province (No. 2016TP1020 ) and the Science and Technology Plan Project of Hengyang City ( 2017 kJ183 ). The work of the second author is supported by Mathematical Research Impact Centric Support (MATRICS) of DST, India ( MTR/2017/000367 ), and the second author is currently at Indian Statistical Institute (ISI), Chennai Centre, Chennai, India. The authors thank the referee for his/her comments.