Add Pytorch-UNet model (#487)
Summary:
Code repo for model: https://github.com/milesial/Pytorch-UNet.git
*Note: I noticed that the model has been changed since the [fork](https://github.com/supriyar/Pytorch-UNet) for the previous attempt to add this model. So I used the (original repository)[https://github.com/milesial/Pytorch-UNet.git].*
Pull Request resolved: https://github.com/pytorch/benchmark/pull/487
Test Plan:
Tested using run.py for all combinations of `{train,eval}` and `{cpu,cuda}` devices and `{jit,eager}` modes
```
Running train method from pytorch_unet on cpu in eager mode
Ran in 3.708993434906006 seconds.
Running train method from pytorch_unet on cuda in eager mode
Ran in 0.06084632873535156 seconds, gpu time 129.12742614746094.
Running train method from pytorch_unet on cpu in jit mode
Ran in 3.7224016189575195 seconds.
Running train method from pytorch_unet on cuda in jit mode
Ran in 0.1113424301147461 seconds, gpu time 180.25369262695312.
Running eval method from pytorch_unet on cpu in eager mode
Ran in 1.3530182838439941 seconds.
Running eval method from pytorch_unet on cuda in eager mode
Ran in 0.00830221176147461 seconds, gpu time 20.900863647460938.
Running eval method from pytorch_unet on cpu in jit mode
Ran in 1.4119977951049805 seconds.
Running eval method from pytorch_unet on cuda in jit mode
Ran in 0.06007719039916992 seconds, gpu time 73.89491271972656.
```
Reviewed By: xuzhao9
Differential Revision: D31401464
Pulled By: rajkar86
fbshipit-source-id: 5274a21f4973a982bf007a503dcb72cd365c11ad
