Fix some mypy issues in functorch/codegen/gen_vmap_plumbing.py

[zou3519]

Stack from ghstack:

• Generate vmap plumbing on all native_functions, not just ones in allowlist #82319
• Stop checking in VmapGeneratedPlumbing.h #82236
• -> Fix some mypy issues in functorch/codegen/gen_vmap_plumbing.py #82314

Necessary to make lint happy in the next PR in the stack

Test Plan

• mypy functorch/codegen/gen_vmap_plumbing.py

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[ezyang]

@pytorchbot merge -g

[pytorchmergebot]

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

[github-actions]

Hey @zou3519.
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.