update types for bundled input and expected output (#748)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/748

this diff updates comments, docs and annotations related to I/O types of bundled program, to make users clearer to know:
1. what kind of type of input can be bundled
2. what kind of type of input can actually be used in the ET method.

Reviewed By: dbort

Differential Revision: D50100273

fbshipit-source-id: 9a6be62a7a83f3b5ac7f478415f30c631dec0825