[docs] Add "how do I" section to torch.package docs (#59503)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59503
**Summary**
This commit adds a "how do I..." section to the `torch.package`
documentation. This section contains short guides about how to solve
real-world problems that frequently recur while using `torch.package`.
**Test Plan**
Continuous integration.
<img width="877" alt="Captura de Pantalla 2021-06-04 a la(s) 9 19 54 p m" src="https://user-images.githubusercontent.com/4392003/120879911-98321380-c57b-11eb-8664-c582c92b7837.png">
Test Plan: Imported from OSS
Reviewed By: Lilyjjo
Differential Revision: D29050629
Pulled By: SplitInfinity
fbshipit-source-id: 2b7800732e0a3c1c947f110c05562aed5174a87f
