[Tests] Add mark skip for GPU

[kylesayrs]

Purpose

• Add GPU mark skip for test_compressor_stacking, which requires a GPU
• Add an explicit GPU test for GHA, so as to unambiguously catch situations where CUDA is not properly installed on a runner

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[dsikka]

Why not update/use existing tools?

llm-compressor/tests/testing_utils.py

Line 40 in cf6a304

def requires_gpu(test_case):

[kylesayrs]

@dsikka If you're referring to here I'm just following the convention of the file. Theoretically we could consolidate these methods at some point, esp if we want to be support amd