Plug the SST2 reader into Forte’s training system

**Is your feature request related to a problem? Please describe.**
This issue is for getting familiar with the Forte pipeline and Texar model.

**Describe the solution you'd like**
Insert the SST2 reader into a Forte pipeline, and train a Texar model.

Start by locating the SST2 reader and the Forte pipeline entry points, then inspect how the Texar model is integrated. Done means the reader runs through a Forte pipeline and a Texar model can be trained on its output, as requested in the issue.

Written by the indexing model from the issue text.