Add CoreML conversion helpers

[ZachNagengast]

This PR supports converting models directly using a new helper function, eg:

from diffusionkit.tests.torch2coreml import (
    convert_mmdit_to_mlpackage,
    convert_vae_to_mlpackage
)

converted_mmdit_path = convert_mmdit_to_mlpackage(
    model_version="stabilityai/stable-diffusion-3-medium", 
    latent_h=64, 
    latent_w=64, 
    output_dir=<your_output_dir>
)

There are a number of folder restructurings as well to support this.

The package is deployed here: https://pypi.org/project/diffusionkit/0.2.15/

[arda-argmax]

Looks good to me! I think we only need to update the Converting Models from PyTorch to Core ML of README.md.

[arda-argmax]

lgtm!