Add support for negative prompt weighting

[sjrummins]

Description

Thank you both so much for the very rapid response to issue #43 with pull request #46!

I think it would be good to support prompt weights on the negative prompt as well. I believe this is supported when not using OpenVINO acceleration.

I think I have been able to implement this myself following the example of #46.

Screenshots/videos:

I've experimented with prompt:

Evening street scene, New York, 1930

and negative prompt:

lamp, (people:1.5)

If I run this without OpenVINO acceleration, I get:

If I run this with OpenVINO acceleration without this code change, I get:

Note that the two are different.

With OpenVINO acceleration with this code change, I get:

This more-or-less matches the non-accelerated version, which suggests to me that the change is working correctly.

Checklist:

• I have read contributing wiki page
• I have performed a self-review of my own code
• My code follows the style guidelines
• My code passes tests

(I have tried to run the tests but they fail with an error "No module named 'gradio'". This happens with or without my code change.)

[sjrummins]

I installed gradio and the tests now pass.

[devangaggarwal]

Great work, @sjrummins ! LGTM, @ynimmaga can you review as well.