Create dataset loader for Abbrev Dataset

## Adding a Dataset
- **Name:** Abbrev Dataset
- **Description:** The Abbrev dataset is made available by Stevenson, et al. (2009). It consists of the acronyms and long-forms from Medline abstracts that were intially prsented by Liu, et al. (2001). The dataset is automatically re-created by identifying the acronyms long froms in Medline and replacing it with it's acronym. The dataset consists of three subsets containing 100, 200 and 300 instances respectively
- **Task:** SPAN_CLASS
- **Paper:** https://aclanthology.org/W09-1309
- **Data:** https://nlp.cs.vcu.edu/data.html
- **License:** ?