Organize the datasets in an convenient module

**Is your feature request related to a problem? Please describe.**
The datasets are currently mixed with the readers. We'd like to separate the datasets and make them easy to be imported.

**Describe the solution you'd like**
Create a module `forte.datasets`, where each sub-module inside will contains enough information for one dataset.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.