Fix an error about the randomness on huggingface demo

[qinghew]

We did not set random seed initially, but the randomly sampled z is same sometimes on our huggingface demo. Our local gradio demo did not meet this issue. So we think this is caused by their online system. Now we set a random seed to sample z every time. The randomness issue seems to be solved. If this issue comes up again, please let us know!