RL: unify rollout generation into one pipeline

[tdene]

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What does this PR do?

Instead of 1 pipeline per environment, this has 1 pipeline total.

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[lauradang]

This is a big change, so approving assuming you have run the gitlab ci/cd tests and maybe a quick test on the cluster if possible (I'm sure you have that going already)

[tdene]

This is a big change, so approving assuming you have run the gitlab ci/cd tests and maybe a quick test on the cluster if possible (I'm sure you have that going already)

This run included these changes: https://wandb.ai/adlr/megatron-rl?nw=9vg5y1477c9

And I've got a more recent one from today as well, if I go dig through wandb.

[svcnvidia-nemo-ci]

🔄 Merge queue validation started!

You can track the progress here: https://github.com/NVIDIA/Megatron-LM/actions/runs/31699429702