fix index generation for optimized hist fc with regression models #5348

Job	Run time
docs	7m 16s
lint	22s
check-examples (03-FFT-examples.ipynb)	3m 12s
tests (macos-13, 3.10, all)	51m 4s
check-examples (04-RNN-examples.ipynb)	7m 1s
tests (ubuntu-latest, 3.10, all)	27m 13s
check-examples (00-quickstart.ipynb)	8m 20s
check-examples (02-data-processing.ipynb)	3m 16s
check-examples (01-multi-time-series-and-covariates.ipynb)	7m 10s
	1h 54m 54s

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