Update tools/launch to take into account number of gpus to avoid reaper

[yfw]

Describe the bug

We have some jobs in our nightly tests (e.g. vlm_grpo-qwen2.5-vl-3b-instruct-clevr-1n2g-dtensor2tp1.v1) that get killed by the Idle Job Reaper because we always request 8 gpus even if we are using less in the run.

https://github.com/NVIDIA-NeMo/RL/blob/fa379fffbc9c5580301fa748dbba269c7d90f883/tools/launch#L165

Steps/Code to reproduce bug

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A helpful guide on on how to craft a minimal bug report http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports.

Expected behavior

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Additional context

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