[export] refactor unflatten.py to be a top-level API

[suo]

Stack from ghstack (oldest at bottom):

• [sigmoid] replace unflatten with upstream version #115468
• [export] Add FlatArgsAdapter to unflatten #115467
• [export] upstream some final fixes to OSS unflatten #115795
• -> [export] refactor unflatten.py to be a top-level API #115466

This is in preparation for the merging of the internal and external versions of
the unflattener. Unflatten needs to be its own API because we are adding more
options to it in forthcoming diffs.

Differential Revision: D52001133

cc @avikchaudhuri @gmagogsfm @zhxchen17 @tugsbayasgalan @angelayi @ydwu4

@diff-train-skip-merge

[pytorch-bot]

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✅ You can merge normally! (1 Unrelated Failure)

As of commit e965048 with merge base 1f3bdf4 ():

UNSTABLE - The following job failed but was likely due to flakiness present on trunk and has been marked as unstable:

• pull / linux-focal-py3.11-clang10 / test (dynamo, 2, 7, linux.2xlarge, unstable) (gh)
  test_linalg.py::TestLinalgCPU::test_linalg_lstsq_cpu_complex128

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[facebook-github-bot]

@pytorchbot merge -f 'Landed internally'

(Initiating merge automatically since Phabricator Diff has merged, using force because this PR might not pass merge_rules.json but landed internally)

[pytorchmergebot]

Merge started

Your change will be merged immediately since you used the force (-f) flag, bypassing any CI checks (ETA: 1-5 minutes). Please use -f as last resort and instead consider -i/--ignore-current to continue the merge ignoring current failures. This will allow currently pending tests to finish and report signal before the merge.

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[pytorchmergebot]

Merge failed

Reason: Command git -C /home/runner/work/pytorch/pytorch cherry-pick -x 66047ff5d56d2f8a13f47d43c75abf9700fe69ad returned non-zero exit code 1

Auto-merging test/export/test_unflatten.py
CONFLICT (content): Merge conflict in test/export/test_unflatten.py
Auto-merging torch/export/exported_program.py
error: could not apply 66047ff5d56... [export] refactor unflatten.py to be a top-level API
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

Details for Dev Infra team

Raised by workflow job

[facebook-github-bot]

@pytorchbot revert -m="Diff reverted internally" -c="ghfirst"

This Pull Request has been reverted by a revert inside Meta. To re-land this change, please open another pull request, assign the same reviewers, fix the CI failures that caused the revert and make sure that the failing CI runs on the PR by applying the proper ciflow label (e.g., ciflow/trunk).)

[pytorchmergebot]

@pytorchbot successfully started a revert job. Check the current status here.
Questions? Feedback? Please reach out to the PyTorch DevX Team

[pytorchmergebot]

Can't find any commits resolving PR