GitLab

Summary:
Pull Request resolved: https://github.com/facebookresearch/d2go/pull/371

In a previous iteration of this diff, we were specifying the teacher model in the same config as the student model, something like:
```
# config.py
MODEL:
  FBNET_V2:
  ...
DISTILLATION:
  TEACHER:
    MODEL:
      FBNET_V2:
      ...
      WEIGHTS: /path/to/teacher/weights
...
```

This leads to some oddities in the code, like we have to have a default config that adds all the required keys in the distillation teacher model.

In this diff, we just let the user supply a teacher config (and optionally runner_name and overwrite opts) and use the supplied runner to build the model:
```
# new_config.py
MODEL:
  FBNET_V2:
...
DISTILLATION:
  TEACHER:
    CONFIG_FNAME: /path/to/teacher/config
    RUNNER_NAME:
...
```

This should make it very easy to specify the teacher as the user could potentially just reuse the trained_config generated in d2go.

Reviewed By: newstzpz

Differential Revision: D37640041

fbshipit-source-id: 088a636c96f98279c9a04e32d1674f703451aec3