Update doc and code to run quantized model (#157)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/157

- Fix doc to separte 1) generating quantized model and b) running it with
  executor_runner
- Include <tuple> in chose_qparams
- Include quantized ops by default in executor_runner

Reviewed By: larryliu0820, guangy10

Differential Revision: D48752106

fbshipit-source-id: 30f4e7ba121abeb01b7b97020c2fef0f5d2ac891