v2.1.0-rc0

Major Updates

• Added SplitMergeTokenizer.
• Add support for token offsets to BertTokenizer.

Minor Updates

• Give BertTokenizer ability to read in a vocab file directly.
• Migrate from std::string to tensorflow::tstring.
• Many build script improvements.
• Update ToDense layer with ragged support attribute.

Bug Fixes

• Update SentencePiece to inherit from TokenizerWithOffsets.
• Fix ICU data linking issue.