Fix bug in `fit_MAP` when shared variables are used in graph (#468) #1728

Job	Run time
windows (windows-latest, 3.12, tests/model)	18m 22s
ubuntu (ubuntu-latest, 3.10, tests/model)	11m 11s
windows (windows-latest, 3.12, tests/statespace/test_statespace.py)	15m 40s
ubuntu (ubuntu-latest, 3.10, tests/statespace/test_statespace.py)	9m 20s
windows (windows-latest, 3.12, tests/statespace/test_kalman_filter.py)	8m 5s
windows (windows-latest, 3.12, tests/statespace --ignore tests/statespace/test_statespace.py --ig...	15m 2s
ubuntu (ubuntu-latest, 3.10, tests/statespace/test_kalman_filter.py)	3m 22s
ubuntu (ubuntu-latest, 3.10, tests/statespace --ignore tests/statespace/test_statespace.py --igno...	8m 4s
windows (windows-latest, 3.12, tests/distributions)	13m 9s
ubuntu (ubuntu-latest, 3.10, tests/distributions)	8m 3s
windows (windows-latest, 3.12, tests --ignore tests/model --ignore tests/statespace --ignore test...	24m 57s
ubuntu (ubuntu-latest, 3.10, tests --ignore tests/model --ignore tests/statespace --ignore tests/...	15m 2s
all_tests	1s
	2h 30m 18s

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