[FBcode->GH] Fix missing kernel guards (#4620)

[datumbox]

This is an old FBcode=>GH commit which was missed. It's also breaking our the linter of main.

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Summary:
Pull Request resolved: #4620

Pull Request resolved: pytorch/nestedtensor#455

Fixes missing kernel guards as identified by D30072495

Reviewed By: jingsh, xush6528

Differential Revision: D31553158

fbshipit-source-id: 80de017ba2ddc52e2a684df9b3eae5de84ed49f4

[facebook-github-bot]

💊 CI failures summary and remediations

As of commit 066bc93 (more details on the Dr. CI page):

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• 1/1 failures introduced in this PR

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1 failure not recognized by patterns:

Job	Step	Action
clang_format	curl https://oss-clang-format.s3.us-east-2.amazonaws.com/linux64/clang-format-linux64 -o clang-format
chmod +x clang-format
sudo mv clang-format /opt/clang-format
./.circleci/unittest/linux/scripts/run-clang-format.py -r torchvision/csrc --clang-format-executable /opt/clang-format
	🔁 rerun

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