[PyTorch][vulkan] Unify convert as `vTensor& convert(const Tensor&)` (#59268)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59268
There's no reason we can't give `convert` this signature: `Tensor::unsafeGetTensorImpl() cocnst ` returns a non-const TensorImpl pointer. (See https://github.com/zdevito/ATen/issues/27#issuecomment-330717839)
ghstack-source-id: 130548716
Test Plan: CI
Reviewed By: SS-JIA
Differential Revision: D28811477
fbshipit-source-id: 269f58980c1f68b29d4be3cba4cd340299ce39af
