Add option to check if frules and rrules are type-inferrable

[sethaxen]

As discussed with @willtebbutt in JuliaDiff/ChainRules.jl#321 (comment), this PR adds a check_inferred=true default keyword argument to the tester functions to check if the frules, rrules, pullbacks, and thunks are all type-stable.

The functionality is there, but I'm having some difficulty testing that test_scalar fails when an rrule or frule is type-unstable. Perhaps a testing kung fu master could give me a pointer.

[willtebbutt]

Hmmm I'm stumped by this one -- not a massive Test.jl expert though. Maybe someone on slack or discourse could assist?

[oxinabox]

The functionality is there, but I'm having some difficulty testing that test_scalar fails when an rrule or frule is type-unstable. Perhaps a testing kung fu master could give me a pointer.

We have a scary, but functional tool for this in
https://github.com/JuliaDiff/ChainRulesTestUtils.jl/blob/master/test/meta_testing_tools.jl

Basically do @test fails(()-><<CODE THAT WILL CALLS A TEST FAILURE>>)
you can see some examples in that file

[oxinabox]

Other than missing the unhappy path tests, this seems good.
It is a breaking change, since it defaults to true, but I think that is fine?

[sethaxen]

It is a breaking change, since it defaults to true, but I think that is fine?

Yes, I think we do want it to be on by default. It is already exposing a fair amount of type-instability in ChainRules. It is a breaking change, and it needs a bump to minor version number.

[sethaxen]

This is ready to merge, but because it bumps the minor version number and will require some changes to ChainRules to bring back compatibility, maybe I should wait for other non-breaking PRs to be merged first?

[willtebbutt]

I have no objection to it being merged, but @oxinabox might want to get a couple of his things in first?

[oxinabox]

yes, I want to get #83 and #84 in first.
and also can we leave this as -DEV as there are some deprecations to be remvoved also

[oxinabox]

This can be merged now