Amend SPIN to be able to handle the cast of rollout_MBS < DP_size

[trias702]

Since we added the generation rollout MBS parameter to SPIN for PR-133, we set it up in such a way that this new rollout MBS cannot be less that the DP size of your model training.

More details can be viewed here

We should now update SPIN to correctly handle the case of rollout MBS < DP size, and this can be done during the upcoming TRTLLM integration work