update compiling-free smoother

[Artiprocher]

Recently, we have received some suggestions from users of DiffSynth. The most important problem is that DiffSynth is not easy to use because it needs to be compiled manually. To address this issue, we implemented a Pythonic version in this repo. Now users only need to install cupy instead of compiling the C++ codes.

[CLAassistant]

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[chywang]

lgtm