`examples/unconditional_image_generation/train_unconditional.py` missing `xformers` support

[vvvm23]

Is your feature request related to a problem? Please describe.
I want the ability to use xformers in the unconditional generation script.

Describe the solution you'd like
Just a copy paste job from the other scripts, to use a flag to turn xformers on. The UNet in the file inherits from ModelMixin so it will already have access to the `enable_xformers_memory_efficient_attention method.

Describe alternatives you've considered
N/A

Additional context
I can make a PR for this 🤗

[pcuenca]

I can make a PR for this 🤗

That'd be awesome, @vvvm23!

[vvvm23]

See above PR!