Keep output specs in sync in ExportedProgramPassBase

[mcremon-meta]

Summary:
A pass that replaces the graph's output node leaves output_specs naming the
node that is no longer there. Nothing catches it at the time; it surfaces later
as a signature that disagrees with the graph, and every pass author has to
remember to fix it up by hand.

ExportedProgramPassBase.__call__ now realigns the specs with the graph between
the pass and the postcondition check:

self.requires(exported_program)
res = self.call(exported_program)
signature_modified = _sync_output_specs(res.exported_program)
self.ensures(res.exported_program)

Rewriting a spec to name the current output node is always safe. Changing an
output between a node and a literal is not, so _sync_output_specs raises
rather than guessing, and a length mismatch between outputs and specs raises
too -- a pass that adds or removes an output is expected to maintain its own
signature.

Two consequences worth calling out:

• A pass that only renames an output now reports modified=True even if it
  returned False, because the signature did change. That is what callers
  driving a pass to fixpoint need to see.
• ensures() now receives the pass's result rather than the program that went
  in. The previous behaviour looks like an oversight -- a postcondition check
  that inspects the pre-pass program cannot check much -- and no pass in the
  tree relies on it: of the subclasses of ExportedProgramPassBase, exactly one
  overrides ensures(), and it calls exported_program.validate(), which wants
  the result.

This was previously implemented as a fused_quant-local subclass. It is not
specific to that package, so it moves here; the subclass goes away in the diff
above this one.

Differential Revision: D120259092

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22864

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 1 New Failure

As of commit e6e2618 with merge base d65efde ():

NEW FAILURE - The following job has failed:

• pull / unittest-editable / windows / windows-job (gh)
  examples/models/test/test_export.py::ExportTest::test_efficient_sam_export_to_executorch

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[meta-codesync]

@mcremon-meta has exported this pull request. If you are a Meta employee, you can view the originating Diff in D120259092.

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.