Fix classifier dropout in AlbertForMultipleChoice
Author: ibraheem-moosa
Pull request: yes

Classification head of AlbertForMultipleChoice uses `hidden_dropout_prob` instead of `classifier_dropout_prob`.  
This is not desirable as we cannot change classifer head dropout probability without changing the dropout 
probabilities of the whole model.

As shown in the paper Albert performance is hurt by dropout. So we should be able to change classifier head probability 
without changing internal dropout of Albert. Also I wonder if changing the internal dropout of a pretrained model is
a good idea or not.

Also I have seen similar issue in Bert and Roberta multiple choice models. I wonder if this is a conscious choice or an
unintended bug.

This PR fixes this behaviour for Albert.

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Fixes # (issue)


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Comments:
Hello! Indeed, thank you for fixing it! We'll gladly welcome PRs that update other models that have this issue.