Update deprecated `gradient_fn` argument to `diff_method` (#1264) #344

Job	Run time
build / compute-build-strategy-matrix	12s
build / validate-inputs	1s
build / validate-poetry-lock-file	19s
build / build-branch (0)	9m 35s
build / aggregate_build	10s
upload-staging	0s
upload-dev / upload	29s
	10m 46s

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