Add GP-VAE/USGAN

[AugustJW]

1. Model description

Add two classific imputation models:

1. GP-VAE(AISTATS'20): a VAE model with Gaussian Process Prior.
2. USGAN(AAAI'21): a GAN model, which is an unsupervised version of SSGAN.

2. Check open-source status

• The model implementation is publicly available

3. Provide useful information for the implementation

No response

[WenjieDu]

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