[BE] Enable Ruff's Flake8 PYI018 (#111101)
Enable [unused-private-type-var (PYI018)](https://docs.astral.sh/ruff/rules/unused-private-type-var/#unused-private-type-var-pyi018)
Link: #110950
Pull Request resolved: https://github.com/pytorch/pytorch/pull/111101
Approved by: https://github.com/albanD
