Implement forward AD for scatter_reduce

[lezcano]

Stack from ghstack:

• -> Implement forward AD for scatter_reduce #85000

I left the case reduction="prod" for future work as it's a bit of a pain.

[pytorch-bot]

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[soulitzer]

Nice!

[soulitzer]

@pytorchbot merge -g

[pytorchmergebot]

@pytorchbot successfully started a merge job. Check the current status here.
The merge job was triggered with the green (-g) flag. This means that your change will be merged once all checks on your PR have passed (ETA: 0-4 Hours). If this is not the intended behavior, feel free to use some of the other merge options in the wiki.
Please reach out to the PyTorch DevX Team with feedback or questions!

[pytorchmergebot]

Merge failed

Reason: Command git -C /home/runner/work/pytorch/pytorch cherry-pick -x 58c4f8e0c6cb679d086cf778573a2ab1d3fbeb4e returned non-zero exit code 1

Auto-merging functorch/test/test_ops.py
CONFLICT (content): Merge conflict in functorch/test/test_ops.py
error: could not apply 58c4f8e0c6... Implement forward AD for scatter_reduce
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

[soulitzer]

Hmm it looks like we may need to manually rebase @lezcano

[lezcano]

@pytorchbot merge

[pytorchmergebot]

@pytorchbot successfully started a merge job. Check the current status here.
The merge job was triggered without a flag. This means that your change will be merged once all checks on your PR have passed (ETA: 0-4 Hours). If this is not the intended behavior, feel free to use some of the other merge options in the wiki.
Please reach out to the PyTorch DevX Team with feedback or questions!

[github-actions]

Hey @lezcano.
You've committed this PR, but it does not have both a 'release notes: ...' and 'topics: ...' label. Please add one of each to the PR. The 'release notes: ...' label should represent the part of PyTorch that this PR changes (fx, autograd, distributed, etc) and the 'topics: ...' label should represent the kind of PR it is (not user facing, new feature, bug fix, perf improvement, etc). The list of valid labels can be found here for the 'release notes: ...' and here for the 'topics: ...'.
For changes that are 'topic: not user facing' there is no need for a release notes label.