multi task roberta
Author: oriyor
Pull request: yes

add a RobertaForMultiTask model.

the model has both RobertaLMHead and RobertaClassificationHead.
when doing the forward pass, we provide an extra 'task' field.

when doing the 'mlm' task, we use RobertaLMHead similarly to RobertaForMaskedLM.

when doing the 'classification' task we use the RobertaClassificationHead, similarly to RobertaForSequenceClassification.

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