ci: tune Nemotron single-GPU model load threads (#3370)

* ci: bound checkpoint CI worker threads

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* fix(ci): limit CPU threads to single-worker jobs

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* fix(ci): scope CPU threads to Nemotron single-GPU

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* chore: log CPU threading configuration

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

* ci: finalize Nemotron thread tuning

Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>

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Signed-off-by: Yuhe Zhang <yuhez@nvidia.com>