[FLAVA] Move masked prediction head to flava_for_pretraining

[ankitade]

Stack from ghstack (oldest at bottom):

• [FLAVA]Separate the pretraining loss from the pretraininig model #278
• -> [FLAVA] Move masked prediction head to flava_for_pretraining #195
• [FLAVA]Move itm head to flava model for pretraining #132
• change order of itm loss init #131

Differential Revision: D38139453

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (gh/ankitade/11/base@60a4486). Click here to learn what that means.
The diff coverage is n/a.

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[ankitade]

@ankitade has imported this pull request. If you are a Facebook employee, you can view this diff on Phabricator.