Add support for `scatter_add` gradient #4

Job	Run time
check-package / init-store	4s
check-schema / schema	11s
precommit-run / pre-commit	25s
check-package / pkg-build (ubuntu-latest)	21s
check-package / pkg-check (ubuntu-latest, 3.10)	1m 20s
check-package / pkg-check (ubuntu-latest, 3.11)	1m 26s
check-package / pkg-check (macOS-latest, 3.10)	1m 4s
check-package / pkg-check (macOS-latest, 3.11)	53s
check-package / pkg-check (windows-latest, 3.10)	3m 17s
check-package / pkg-check (windows-latest, 3.11)	3m 10s
check-package / pkg-guardian	1s
	12m 12s

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