Update clip_back.py

[lizhiustc]

Return image embeddings

[rom1504]

A few things to do here:

• Put it under an option of the API so by default it doesn't get returned
• check if the serialization is good
• Run it (eg on a small index) to make sure it works

Could you do that ?

[lizhiustc]

It's a little difficult and may take a few days. But I'll try my best.

[rom1504]

Why close ?