[vulkan] Remove external dependencies in core API and introduce torch_vulkan_api target (#91021)
This diff isolates the core components of the Pytorch Vulkan backend into its own target (`//xplat/caffe2:torch_vulkan_api`). The main motivation for this is to create a library that does not have a dependency on the ATen library which can then be used to build a graph mode runtime for Vulkan for Executorch.
In addition to introducing the new target, this diff also removes some references to external dependencies in the `api/` folder so that files in that folder are completely self contained.
Differential Revision: [D42038817](https://our.internmc.facebook.com/intern/diff/D42038817/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D42038817/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91021
Approved by: https://github.com/kirklandsign
