revert back to saving and loading a TFM with torch rather than pickle #5536

Job	Run time
docs	7m 33s
check-examples (03-FFT-examples.ipynb)	3m 4s
lint	24s
check-examples (04-RNN-examples.ipynb)	7m 0s
tests (macos-13, 3.10, all)	39m 39s
check-examples (00-quickstart.ipynb)	7m 46s
tests (ubuntu-latest, 3.10, all)	31m 11s
check-examples (02-data-processing.ipynb)	3m 29s
check-examples (01-multi-time-series-and-covariates.ipynb)	8m 34s
	1h 48m 40s

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