Make nemo.collections.llm PreTrainingDataModule num samples configurable
[🤖]: Hi @hemildesai 👋,

We wanted to let you know that a [CICD pipeline](https://github.com/NVIDIA/NeMo/actions/runs/11625066114) for this PR just finished successfully

So it might be time to merge this PR or get some approvals

I'm just a bot so I'll leave it you what to do next.

//cc @pablo-garay @ko3n1g
This is so that users can build a larger cache and reuse it across different runs.