Add ntasks for older slurm versions.

Without this flag, on slurm 23.11.10 we get this error:

```
sbatch: error: Failed to validate job spec. --gpus-per-task or --tres-per-task used without either --gpus or -n/--ntasks is not allowed.
sbatch: error: Invalid generic resource (gres) specification
```

Author: ahmadsharif1

torchx/schedulers/slurm_scheduler.py

@@ -210,6 +210,7 @@ def from_role(
                     sbatch_opts.setdefault("gpus-per-node", str(resource.gpu))
                 else:
                     sbatch_opts.setdefault("gpus-per-task", str(resource.gpu))
+                    sbatch_opts.setdefault("ntasks", "1")
 
         srun_opts = {
             "output": f"slurm-{macros.app_id}-{name}.out",