Integrate more task-specific encoders

[chen-yuxuan]

Integrate the following task-specific models as encoders:

• bert-english-uncased-finetuned-pos (named as bert-pos)
• bert-base-uncased-MNLI (named as bert-mnli)
• bert-base-cased-squad2 (named as bert-squad)
• roberta-base-squad2 (named as roberta-squad)

Results on CoNLL 2003 are updated on OverLeaf.