chore: Set default return type to ExportedProgram

[peri044]

Description

This change adds the following

1. Returns ExportedProgram as the output of compilation by default
2. output_format flag to return desired output format (exported_program | torchscript | graph_module)
3. Modify export serde test cases to handle the above changes

Fixes # (issue)

Type of change

Please delete options that are not relevant and/or add your own.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project (You can use the linters)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas and hacks
• I have made corresponding changes to the documentation
• I have added tests to verify my fix or my feature
• New and existing unit tests pass locally with my changes
• I have added the relevant labels to my PR in so that relevant reviewers are notified

[github-actions]

Code conforms to C++ style guidelines

[github-actions]

Code conforms to Python style guidelines

[gs-olive]

Overall looks good. Added a question, and it seems 2 tests are failing currently. Consider redirecting the branch to the staging_rel_2_2 branch to get the release test suite.

[narendasan]

We need documentation updates in this PR

[peri044]

This PR has an open issue pytorch/pytorch#116831 which should be fixed first

[github-actions]

Code conforms to C++ style guidelines

[github-actions]

Code conforms to Python style guidelines

[github-actions]

Code conforms to Python style guidelines

[github-actions]

Code conforms to C++ style guidelines

[gs-olive]

Overall, looks great - just added a few questions on the exporter.