Hotfix: fix `.diagonal()` calls for keops kernel matrices. #668

Job	Run time
run_linter	35s
run_unit_tests (master, with-extras)	3m 7s
run_unit_tests (master, no-extras)	2m 42s
run_examples	5m 18s
run_unit_tests (stable, with-extras)	2m 21s
run_unit_tests (stable, no-extras)	1m 35s
	15m 38s

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