make stable diffusion unet and vae number of channels static

[eaidova]

What does this PR do?

Ported from huggingface/optimum-intel#692
remove number of channels from unet and vae dynamic inputs and outputs. From model topology perspective, this value can not be different during inference from specified in config (changing number of channels requires to recreate pytorch model as number of channels in the first conv depends on it). Having them static will simplify model graph

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

@echarlaix

[echarlaix]

LGTM !

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.