[examples] consistently use --gpus, instead of --n_gpu
- some docs were wrongly suggesting to use `--n_gpu`, when the code is `--gpus`
- `examples/distillation/` had  `--n_gpu`, in the code - switched it and the doc to `--gpus`
I had to update all the files since n_gpu seems like a common param.<|||||>You could create an issue, suggesting to use `n_gpu` everywhere instead, supporting it with some stats that would be in favor of this naming. As long as it's consistent across the project either way works, IMHO.