[vulkan][ez] fix always printing out a warning when retrieving the global context (#86697)
Summary: D40151818 (https://github.com/pytorch/pytorch/commit/82ed5ca3401e965067fd03a6bac57978f884f715) replaces the `TORCH_CHECK` with a `TORCH_WARN` but since it does not check if the context is valid the message gets printed every time. This diff fixes that.
Test Plan:
Referring to [Pytorch Vulkan Testing Procedures](https://fb.quip.com/fZALAc9zhlcU)
On Mac:
1. `vulkan_api_test` on Mac
2. model comparison binary on Mac
On Android:
1. `vulkan_api_test` on Android
2. benchmark binary on Android
Reviewed By: salilsdesai
Differential Revision: D40266820
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86697
Approved by: https://github.com/kirklandsign
