Add stable diffusion unit test (#2496)

Add a unit test for Stable Diffusion without using stable-diffusion
model that needs HF token.

Midjourney model does not need the HF token and has the same structure.

We verified this by printing both models. The following is the
structure:

``` StableDiffusionPipeline {
  "_class_name": "StableDiffusionPipeline",
  "_diffusers_version": "0.7.2",
  "feature_extractor": [
    "transformers",
    "CLIPFeatureExtractor"
  ],
  "safety_checker": [
    "stable_diffusion",
    "StableDiffusionSafetyChecker"
  ],
  "scheduler": [
    "diffusers",
    "PNDMScheduler"
  ],
  "text_encoder": [
    "transformers",
    "CLIPTextModel"
  ],
  "tokenizer": [
    "transformers",
    "CLIPTokenizer"
  ],
  "unet": [
    "diffusers",
    "UNet2DConditionModel"
  ],
  "vae": [
    "diffusers",
    "AutoencoderKL"
  ]
}
```
@cmikeh2

---------

Co-authored-by: Olatunji Ruwase <olruwase@microsoft.com>
Co-authored-by: Logan Adams <114770087+loadams@users.noreply.github.com>
Co-authored-by: Logan Adams <loadams@microsoft.com>
Co-authored-by: Lev Kurilenko <lekurile@microsoft.com>
Co-authored-by: Lev Kurilenko <113481193+lekurile@users.noreply.github.com>