[vulkan] Fix warnings: static_cast, remove unused (#42195)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/42195
Test Plan: Imported from OSS
Reviewed By: AshkanAliabadi
Differential Revision: D22803035
Pulled By: IvanKobzarev
fbshipit-source-id: d7bf256437eccb5c421a7fd0aa8ec23a8fec0470
