[LLM] Rewrite GSM8K reward function to follow standard GRPO conventions #8552

Job	Run time
tests-gpu-distributed (3.12, 13.0) / linux-job	29m 9s
tests-olddeps (3.10, 11.8) / linux-job	1h 0m 0s
test-setup-minimal (3.10) / linux-job	5m 52s
tests-gpu (3.12, 13.0, 2) / linux-job	16m 31s
tests-optdeps (3.12, 13.0) / linux-job	58m 54s
tests-gpu (3.12, 13.0, 1) / linux-job	12m 44s
tests-gpu (3.12, 13.0, 3) / linux-job	11m 40s
test-setup-minimal (3.14) / linux-job	5m 37s
tests-stable-gpu-distributed (3.12, 13.0) / linux-job	29m 36s
tests-cpu (3.11) / linux-job	51m 23s
tests-cpu (3.12) / linux-job	53m 48s
tests-cpu (3.10) / linux-job	52m 58s
tests-cpu (3.14) / linux-job	35m 54s
tests-cpu (3.13) / linux-job	50m 31s
tests-stable-gpu (3.12, 13.0, 1) / linux-job	12m 41s
tests-stable-gpu (3.12, 13.0, 3) / linux-job	11m 27s
tests-stable-gpu (3.12, 13.0, 2) / linux-job	16m 23s
upload-codecov	13s
	8h 35m 21s

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