Sentencepiece version upgrade

[rkoystart]

Hi @levyfan , the current version of sentencepiece used in this project is 0.1.96. The latest version available is 0.2.0. I tried upgrading the version in the CMakeLists.txt which lead to many errors because the 0.2.0 sentencepiece version required cxx 17 leading to few data type mismatches (std::basic_string).

Is there any possibility of upgrading the latest version in this repo as well ?