Fix step size calculation if some results are non-deterministic (inf …

[wsmoses]

…derivative) or nan

step size is calculated based of the initial derivatives of all results. However, if some of the results return nan and/or are non-determinstic, the step size will be inf/nan, causing all other results to be broken -- instead of just the result which is expected to have an inf/nan derivative

cc @vchuravy @giordano

[wsmoses]

relatedly @oscardssmith can we add EnzymeTestUtils as reverse CI here too [as I see ChainRulesTestUtils as such a a reverse CI]

[oscardssmith]

I would defer on CI decisions to someone more deeply involved in this package.

[wsmoses]

fair and yeah can follow up later on that.

ci chainrules error is unrelated

[oscardssmith]

I'm not aware of who actually has merge rights here. Looks like the most recent PRs merged were last year by @oxinabox, and @wesselb, so presumably this should get a review from one of them.