Fix bug when converting checkpoint to diffusers format

[ehsan]

What does this PR do?

Fixes a regression in using the yaml module for reading the config file when converting a checkpoint to diffusers format. If no original config file is passed, original_config_file is set to a config yaml string. This PR updates the else branch to do the same thing there.

Fixes #6899

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Who can review?

@sayakpaul

[sayakpaul]

Thanks!

Do you also have an example command that successfully runs with this PR?

[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.

[ehsan]

Do you also have an example command that successfully runs with this PR?

For reference:

python3 /path/to/diffusers/scripts/convert_original_stable_diffusion_to_diffusers.py   --pipeline_class_name StableDiffusionXLPipeline  --checkpoint_path  /path/to/file.ckpt --original_config_file /path/to/config.yaml --dump_path /path/to/output