observer bench: add CUDA (#39360)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39360
Makes the observer microbenchmarks also run on CUDA. This is useful
now that QAT is supported in DDP and is more likely to be run
on GPUs.
Test Plan:
```
python -m pt.qobserver_test
```
Imported from OSS
Differential Revision: D21828985
fbshipit-source-id: 6da4d61f744f7a2ee5e87963b3ec84579128d435
