Fix export config in examples (#34)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/34

## Context
It doesn't look correct that want to export models with one config but test the export with a different one.

## This DIff
 - Ensure canonical config is used for both export and tests
 - Ensure the test is loading the lowered model through `_load_for_executorch_from_buffer` for output consistency checking
 - Remove confusion about certain export flags by adding more inline comments to those flags, e.g. why  is `enable_dynamic_shape` there not enabled by default, what does `enable_aot` do with dynamic shapes, etc.

Reviewed By: JacobSzwejbka

Differential Revision: D48018569

fbshipit-source-id: 5e0696c19dea539dec5c23ac3566199bd2ce981f