Ignore lchoose in expression tests

[rok-cesnovar]

Summary

stan-dev/stanc3#1010 will add lchoose for vectors, row vectors and arrays. Because lchoose is generated as binomial_coefficient_log in stanc3 we need to add an exception to the expressions tests.

Tests

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Side Effects

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Checklist

• Copyright holder: Rok Češnovar

[WardBrian]

LGTM

Why did this pass tests before? Lchoose was still around before the change

[rok-cesnovar]

Because expression templating is used with Eigen inputs (vectors, row vectors, matrices) and signatures without those inputs do not require testing expressions (previously it was only scalars I believe?)

[WardBrian]

That makes sense. I wonder if this is why the signature was left un vectorized, the tests failed and they just changed it back to scalars only

[rok-cesnovar]

I doubt it, expressions have been live for 2 release cycles so the expressions testing was added then as well and I dont remember seeing a lchoose PR and I have been following them quite closely in the last year or so. Always an option I missed stuff though :)

[WardBrian]

Running the full tests on this seems rather unnecessary, doesn't it?

[rok-cesnovar]

Yeah, probably true, though sig_utils is used in expressions tests in Math as well so technically its good if we run it. We could force it to just run expressions tests if the only change is in this file, but not sure its worth the hassle for an edge case.

The bigger issue is #2324 (comment)

Given that expressions tests passed we can go ahead and merge it though.