Refactor model classes (facebookresearch#180)

Summary:
Pull Request resolved: facebookresearch#180

Create new classes for phrase grounding and VQA tasks. Move contrastive alignment
projections out of the core model. Add new loss classes for VQA.

Test Plan:
Note that we modified the test results in test_mdetr, but it is just a revert to the numbers
prior to PR facebookresearch#138 (where we initially added the contrastive alignment projections to the model class).
```
python -m pytest -vv
```
E2E test in subsequent PRs

Reviewed By: RdoubleA

Differential Revision: D38160122

Pulled By: ebsmothers

fbshipit-source-id: 569643a35b15764d3f2dabec2026eee97c3ef251