Fix bug in `fit_MAP` when shared variables are used in graph #1726

Job	Run time
windows (windows-latest, 3.12, tests/model)	19m 14s
ubuntu (ubuntu-latest, 3.10, tests/model)	11m 22s
windows (windows-latest, 3.12, tests/statespace/test_statespace.py)	15m 10s
ubuntu (ubuntu-latest, 3.10, tests/statespace/test_statespace.py)	9m 12s
windows (windows-latest, 3.12, tests/statespace/test_kalman_filter.py)	7m 25s
ubuntu (ubuntu-latest, 3.10, tests/statespace/test_kalman_filter.py)	3m 27s
windows (windows-latest, 3.12, tests/statespace --ignore tests/statespace/test_statespace.py --ig...	15m 8s
ubuntu (ubuntu-latest, 3.10, tests/statespace --ignore tests/statespace/test_statespace.py --igno...	8m 38s
windows (windows-latest, 3.12, tests/distributions)	14m 28s
ubuntu (ubuntu-latest, 3.10, tests/distributions)	7m 58s
windows (windows-latest, 3.12, tests --ignore tests/model --ignore tests/statespace --ignore test...	24m 44s
ubuntu (ubuntu-latest, 3.10, tests --ignore tests/model --ignore tests/statespace --ignore tests/...	14m 54s
all_tests	1s
	2h 31m 41s

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