Add PyTorch platform handler example (triton-inference-server#287) (triton-inference-server#289)

* Add PyTorch platform handler example

* Refactor docs structure

* Add more comments and minor refactoring

* Further break down client.py

* Remove exit 0 if terminated normally

* Simplify comments

* Improve comment

* List mug.jpg paths

* Docs update

* Describe the source of mug.jpg

Co-authored-by: Jacky <[email protected]>

Author: tanmayv25