Add deeplab_v3 model to examples (#60)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/60

Add image segmentation model **deeplabv3_resnet50** to `executorch/examples`.

Info about the model: https://pytorch.org/hub/pytorch_vision_deeplabv3_resnet101/

Reviewed By: kirklandsign

Differential Revision: D48136966

fbshipit-source-id: 62e91450eeccf61adce8657f50a9f618b2be087e