feat: copy and refacotor the code from DDSP_SVC

[shdancer]

I see the feature request from issue #762. Then, I referenced the newest code from DDSP-SVC 4.0 and refactored the DDSP-SVC code. Here are several changes and the reasons:

According to the recommendation of DDSP-SVC, there's no need to pass the DDSP model; only the Diff model is required. So, I changed the frontend code to allow the model to be empty.
Referring to gui_diff.py in DDSP-SVC's repository, I modified the settings of the DDSP-SVC voice changer.
Known bugs:
When the chunk size is too small, it seems that DDSP-SVC itself will encounter an error related to f0_extraction.

This is my first time doing a PR. If @w-okada can help me merge, I would be really apppreciated for that.

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[shdancer]

I have read the CLA Document and I hereby sign the CLA

[w-okada]

we cannot use praat-parselmouth because of license issue.

[shdancer]

So if I remove the praat-parselmouth, will it be safe? In fact, the project is not using parselmouth as a f0_extractor besiedes.

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Already remove praat-parselmout @w-okada

[yxlllc]

So if I remove the praat-parselmouth, will it be safe? In fact, the project is not using parselmouth as a f0_extractor besiedes.

Already remove praat-parselmout @w-okada

ddsp-svc is using rmvpe as default f0 extractor now.