Summary:
I found that the json annotations for Pascal VOC downloaded from the [URL](https://storage.googleapis.com/coco-dataset/external/PASCAL_VOC.zip) in [detectron/datasets/data/README.md](https://github.com/facebookresearch/Detectron/blob/master/detectron/datasets/data/README.md#pascal-voc-annotations-in-coco-format) are split into three sets: 'train', 'val' and 'test'.
Therefor, it's unable to use 'trainval' set directly.
For example, the code can't found the 'voc_2007_trainval.json' file when using the following configure:
```python
TRAIN:
  DATASETS: ('voc_2007_trainval',)
Closes https://github.com/facebookresearch/Detectron/pull/299

Differential Revision: D8202450

Pulled By: ir413

fbshipit-source-id: 8d8bc8ce406b2d2ed4af57953846d21fa1968018