Add out stream parameter to pretty_print and print_program (#3397)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3397

Add an optional TextIO parameter for redirecting the prints to a file or buffer
for the `pretty_print` and `print_program` functions.
Default is to write to stdout as before, so no changes should occur
for any existing users.

Reviewed By: Vysarat, dbort

Differential Revision: D56711767

fbshipit-source-id: 818831c06f07aee0173064a8621a8e2b861acb6a