Fix multi grad hooks thread safety

[soulitzer]

Stack from ghstack (oldest at bottom):

• -> Fix multi grad hooks thread safety #132055

Thanks @awgu for spotting this

[pytorch-bot]

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[soulitzer]

@pytorchbot merge

[pytorchmergebot]

Merge failed

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[soulitzer]

@pytorchbot merge

[pytorchmergebot]

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[pytorchmergebot]

Merge failed

Reason: 1 mandatory check(s) failed. The first few are:

• Lint / lintrunner-noclang / linux-job

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Failing merge rule: Core Maintainers

[awgu]

@pytorchbot merge

[pytorchmergebot]

Merge started

Your change will be merged once all checks pass (ETA 0-4 Hours).

Learn more about merging in the wiki.

Questions? Feedback? Please reach out to the PyTorch DevX Team

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