[tests] introduce `VAETesterMixin` to consolidate tests for slicing and tiling #1609

Job	Run time
check_code_quality	32s
check_repository_consistency	35s
Setup Torch Pipelines CUDA Slow Tests Matrix	2m 19s
Torch CUDA Tests (lora)	13m 34s
Torch CUDA Tests (schedulers)	3m 35s
Torch CUDA Tests (models)	7m 46s
Examples PyTorch CUDA tests on Ubuntu	43m 37s
Torch CUDA Tests (others)	1m 20s
Torch Pipelines CUDA Tests (controlnet_flux)	4m 1s
Torch Pipelines CUDA Tests (ip_adapters)	3m 23s
Torch Pipelines CUDA Tests (controlnet_sd3)	4m 0s
Torch Pipelines CUDA Tests (stable_diffusion_xl)	6m 1s
Torch Pipelines CUDA Tests (stable_diffusion_3)	4m 3s
Torch Pipelines CUDA Tests (flux)	5m 18s
Torch Pipelines CUDA Tests (controlnet)	8m 55s
Torch Pipelines CUDA Tests (stable_diffusion)	5m 1s
Torch Pipelines CUDA Tests (stable_diffusion_2)	5m 3s
	1h 59m 3s

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