Make an option to run task groups sequentially instead of in-parallel

[Kipok]

Would be great to add "sequential" option to task groups, such that they can be run one after another instead of in-parallel. The usecase for this is that sometimes we need to run a couple of conversion steps, e.g. from nemo -> hf and then from hf -> trtllm and these steps need different containers. We also sometimes need to run inference and then do evaluation in different containers where evaluation is only taking a small time. Currently we run those steps by launching separate tasks, which will request separate nodes and thus might stay in slurm queue longer. Would be great to have an option to have multiple sequential sruns in the same sbatch script so that we can use different containers, but reuse the node.

[hemildesai]

I will pick this up next.