[Feature] Add reduction parameter to On-Policy losses. #611

Job	Run time
unittests-atari-dqn	0s
unittests-d4rl	0s
unittests-envpool	0s
unittests-gendgrl	0s
unittests-minari	0s
unittests-openx	0s
unittests-roboset	0s
unittests-smacv2	0s
unittests-vd4rl	0s
unittests-pettingzoo / linux-job	9m 0s
unittests-brax (3.9, 12.1) / linux-job	12m 14s
unittests-gym (3.9, 12.1) / linux-job	17m 19s
unittests-jumanji (3.9, 12.1) / linux-job	8m 27s
unittests-robohive (3.9, 12.1) / linux-job	9m 59s
unittests-sklearn (3.9, 12.1) / linux-job	5m 44s
unittests-vmas (3.9, 12.1) / linux-job	13m 6s
	1h 15m 49s

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