Fix installation for diffuser_instruct_pix2pix (#2184)
Summary:
Currently `diffuser_instruct_pix2pix/install.py` tries to import `diffusers` before installing the requirements throwing an error:
```
from diffusers import StableDiffusionInstructPix2PixPipeline
ModuleNotFoundError: No module named 'diffusers'
```
These changes fix the installation making this file similar to other diffuser installation scripts.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/2184
Reviewed By: aaronenyeshi
Differential Revision: D54884958
Pulled By: xuzhao9
fbshipit-source-id: 5a80b2d76879ca095b8e12151535983864876805
