Batched inference on images using DensePose?

## ❓ How to do something using detectron2
Currently, DensePose reads in single images and infer dense annotations. This is very slow and quite wasteful. Does DensePose have the ability to read in batches of images to perform inference?

Describe what you want to do, including:
1. what inputs you will provide, if any:
A video filled with images

2. what outputs you are expecting:
A pickle file with dense pose annotations, except inferred a lot faster.

## ❓ What does an API do and how to use it?
Please link to which API or documentation you're asking about from
https://detectron2.readthedocs.io/

NOTE:

1. Only general answers are provided.
If you want to ask about "why X did not work", please use the
[Unexpected behaviors](https://github.com/facebookresearch/detectron2/issues/new/choose) issue template.

2. About how to implement new models / new dataloader / new training logic, etc., check documentation first.

3. We do not answer general machine learning / computer vision questions that are not specific to detectron2, such as how a model works, how to improve your training/make it converge, or what algorithm/methods can be used to achieve X.