[Feature] inline `hold_out_net` #2911

Job	Run time
unittests-meltingpot / linux-job	11m 48s
unittests-pettingzoo / linux-job	9m 34s
unittests-atari-dqn (3.9, 12.1) / linux-job	12m 33s
unittests-brax (3.11, 12.1) / linux-job	10m 34s
unittests-d4rl (3.9, 12.1) / linux-job	8m 48s
unittests-envpool (3.9, 12.1) / linux-job	9m 22s
unittests-gendgrl (3.9, 12.1) / linux-job	7m 33s
unittests-gym (3.9, 12.1) / linux-job	18m 45s
unittests-jumanji (3.9, 12.1) / linux-job	33m 41s
unittests-minari (3.9, 12.1) / linux-job	6m 17s
unittests-unity_mlagents (3.10.12, 12.1) / linux-job	8m 33s
unittests-open_spiel (3.9, 12.1) / linux-job	7m 57s
unittests-openx (3.9, 12.1) / linux-job	18m 33s
unittests-robohive (3.9, 12.1) / linux-job	46m 41s
unittests-roboset (3.9, 12.1) / linux-job	10m 17s
unittests-sklearn (3.9, 12.1) / linux-job	6m 4s
unittests-smacv2 (3.9, 12.1) / linux-job	16m 45s
unittests-vd4rl (3.9, 12.1) / linux-job	9m 46s
unittests-vmas (3.9, 12.1) / linux-job	11m 59s
	4h 25m 30s

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