add a new method to model_manager that retrieves individual pipeline components

[lstein]

This PR introduces a new set of ModelManager methods that enables you to retrieve the individual parts of a stable diffusion pipeline model, including the vae, text_encoder, unet, tokenizer, etc.

To use:

from invokeai.backend import ModelManager

manager = ModelManager('/path/to/models.yaml')

# get the VAE
vae = manager.get_model_vae('stable-diffusion-1.5')

# get the unet
unet = manager.get_model_unet('stable-diffusion-1.5')

# get the tokenizer
tokenizer = manager.get_model_tokenizer('stable-diffusion-1.5')

# etc etc
feature_extractor = manager.get_model_feature_extractor('stable-diffusion-1.5')
scheduler = manager.get_model_scheduler('stable-diffusion-1.5')
text_encoder = manager.get_model_text_encoder('stable-diffusion-1.5')

# if no model provided, then defaults to the one currently in GPU, if any
vae = manager.get_model_vae()

[blessedcoolant]

Code reads fine to me. Pretty straight forward. Do I need to test any use cases here?

[lstein]

Code reads fine to me. Pretty straight forward. Do I need to test any use cases here?

I don't think so. I ran through all the test cases and didn't find any problems.