Add auto-populate task labels based on data

[prasang-gupta]

Implemented changes primarily in shared/const.py to enable auto-populating NER and REL labels based on data (apart from the standard labels present for ace04, ace05 and scierc). Changes include:

• Auto-populating labels when task is not one of the 3 standards (Removal of choice in task parameter)
• Storing the new-found labels in output dir at the time of entity model run for relation and relation-approx models
• Converted task labels into a class for better management
• Using the pre-populated labels in case task belongs to one of the 3 standards (for consistent replication of paper results)