The ability to choose how to dump DensePose output

## 🚀 Feature
DensePose saves intermediate results in memory. This becomes very difficult to run on a large set of images. It requires us to fragment into smaller chunks which is just painful to manage.

## Motivation & Examples
Provide an example to instead save out result as individual files for each image.

The issue names DensePose output and the need to avoid keeping intermediate results in memory, but it does not identify files, tests, or an entry point. Start by tracing where DensePose results are accumulated and dumped; done means supporting per-image output files and providing an example of that usage.

Written by the indexing model from the issue text.