Fix bug in `fit_MAP` when shared variables are used in graph #1721

Job	Run time
ubuntu (ubuntu-latest, 3.10, tests/model)	11m 17s
windows (windows-latest, 3.12, tests/model)	18m 11s
ubuntu (ubuntu-latest, 3.10, tests/statespace/test_statespace.py)	9m 28s
ubuntu (ubuntu-latest, 3.10, tests/statespace/test_kalman_filter.py)	3m 42s
ubuntu (ubuntu-latest, 3.10, tests/statespace --ignore tests/statespace/test_statespace.py --igno...	8m 58s
windows (windows-latest, 3.12, tests/statespace/test_statespace.py)	15m 51s
ubuntu (ubuntu-latest, 3.10, tests/distributions)	8m 3s
windows (windows-latest, 3.12, tests/statespace/test_kalman_filter.py)	6m 34s
ubuntu (ubuntu-latest, 3.10, tests --ignore tests/model --ignore tests/statespace --ignore tests/...	14m 43s
windows (windows-latest, 3.12, tests/statespace --ignore tests/statespace/test_statespace.py --ig...	15m 18s
windows (windows-latest, 3.12, tests/distributions)	14m 49s
windows (windows-latest, 3.12, tests --ignore tests/model --ignore tests/statespace --ignore test...	25m 16s
all_tests	1s
	2h 32m 11s

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