Allow stem feature map extraction
Allow stem feature map extraction

Summary:
allow users to extract the 'stem' feature map for downstream tasks by simply specifying `stem` in the `MODELS.RESNETS.OUT_FEATURES`.

Example usage
```
BACKBONE:
    NAME: "build_resnet_backbone"
  WEIGHTS: "detectron2://ImageNetPretrained/MSRA/R-50.pkl"
  RESNETS:
    DEPTH: 50
    OUT_FEATURES: ["stem", "res2", "res3", "res4", "res5"]
```

Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/2100

Reviewed By: alexander-kirillov

Differential Revision: D24114854

Pulled By: ppwwyyxx

fbshipit-source-id: 421e0172f3b0774613af5b06fe7e2b04765d9de4

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2