Mark Vulkan test as unstable (#98106)
While investigating the new flaky issue in trunk, i.e. https://hud.pytorch.org/pytorch/pytorch/commit/f9ca48ddb58196fa2003d6253c38c84567893e34. Curiously, this starts to become flaky after https://github.com/pytorch/pytorch/pull/97698, so I'll reach out to the author.
Fixes https://github.com/pytorch/pytorch/issues/98071
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98106
Approved by: https://github.com/clee2000
