turn on -Werror=unused-but-set-variable

[dagitses]

Stack from ghstack (oldest at bottom):

• turn on -Werror=extra in Bazel #79327
• disable warnings for external repositories #79345
• turn on -Wall with a few exceptions in Bazel build #79306
• -> turn on -Werror=unused-but-set-variable #79305
• move CUDA flags out of --per_file_copts into the cu_library macro #79414
• precisely specify the regexps of our unused-function exemptions #79313
• quote all of the regular expressions #79312
• avoid multiline strings in our warning flag overrides #79308

Summary:
Also fix the one violation.

Test Plan:

Reviewers:

Subscribers:

Tasks:

Tags:

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[dagitses]

@pytorchbot merge --green

[pytorchmergebot]

@pytorchbot successfully started a merge job. Check the current status here

[dagitses]

@pytorchbot merge --green

[pytorchmergebot]

@pytorchbot successfully started a merge job. Check the current status here

[pytorchmergebot]

Merge failed due to Command git -C /home/runner/actions-runner/_work/pytorch/pytorch cherry-pick -x 13f0a99a7668be858f355a67e453f940f973f577 returned non-zero exit code 1

Auto-merging .bazelrc
CONFLICT (content): Merge conflict in .bazelrc
Auto-merging tools/rules/cu.bzl
CONFLICT (content): Merge conflict in tools/rules/cu.bzl
error: could not apply 13f0a99a76... turn on -Werror=unused-but-set-variable
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".

Raised by https://github.com/pytorch/pytorch/actions/runs/2490016002

[dagitses]

@pytorchbot merge --green

[pytorchmergebot]

@pytorchbot successfully started a merge job. Check the current status here

[github-actions]

Hey @dagitses.
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.