Match the number of outputs of backward with forward for AllGatherGrad

[ArvinZhuang]

What does this PR do?

Fixes #6624

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

[tchaton]

Nice catch !

[Borda]

@ArvinZhuang nice, can you also add a test to catch this issue later, I would say that the example from issue shall be fine :]

[ArvinZhuang]

Hi @Borda, I have added a test that raises the error before the fix and passes after.

[awaelchli]

thanks for the fix

[awaelchli]

this won't run
we need to add add it to tests/special_tests.sh
special=True means this test gets skipped by default
and skip_windows would be redundant in that case.

[ArvinZhuang]

Yeah, I noticed this, I actually commented out special=True when I test this. I put it here just because I was following the same settings of other tests in this script.