[tests] introduce `VAETesterMixin` to consolidate tests for slicing and tiling #1607

Job	Run time
check_code_quality	33s
check_repository_consistency	28s
Setup Torch Pipelines CUDA Slow Tests Matrix	2m 27s
Torch CUDA Tests (schedulers)	3m 36s
Torch CUDA Tests (models)	7m 41s
Torch CUDA Tests (lora)	11m 34s
Torch CUDA Tests (others)	1m 20s
Examples PyTorch CUDA tests on Ubuntu	42m 53s
Torch Pipelines CUDA Tests (controlnet)	9m 27s
Torch Pipelines CUDA Tests (flux)	3m 12s
Torch Pipelines CUDA Tests (controlnet_flux)	1m 53s
Torch Pipelines CUDA Tests (ip_adapters)	1m 8s
Torch Pipelines CUDA Tests (stable_diffusion_2)	2m 55s
Torch Pipelines CUDA Tests (stable_diffusion_xl)	4m 8s
Torch Pipelines CUDA Tests (stable_diffusion_3)	1m 59s
Torch Pipelines CUDA Tests (controlnet_sd3)	1m 53s
Torch Pipelines CUDA Tests (stable_diffusion)	3m 8s
	1h 40m 15s

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