[pytorch][vulkan] Fix downcast warnings-errors, aten_vulkan buck target
Summary:
buck build has -Wall for downcasts - need to add safe_downcast<int32_t> everywhere
BUCK build changes for aten_vulkan to include vulkan_wrapper lib
Test Plan: The next diff with segmentation demo works fine
Reviewed By: dreiss
Differential Revision: D23739445
fbshipit-source-id: b22a30e1493c4174c35075a68586defb0fccd2af
