Add `job_index` variable to `simple_hypersearch`

In certain situations it might be useful to access the index of a job when generating commands using the `simple_hypersearch` script. This would for example allow the definition of unique output paths for different hyperparameter search evaluations.

For example:
```bash
simple_hypersearch "python train_model.py --lr {lr} --bs {bs} --output hypersearch/{job_index}" -p lr 0.001 0.0001 -p bs 32 64 128
```