handle fp16 in `UNet2DModel` (#1216)
* make sure fp16 runs well
* add fp16 test for superes
* Update src/diffusers/models/unet_2d.py
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>
* gen on cuda
* always run fast inferecne test on cpu
* run on cpu
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>
