Fixes 975 by only removing leftmost array dimension if equal to 1

[SteveBronder]

Submission Checklist

• Run unit tests
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Summary

Fixes #975 by removing only the leftmost dimension of the init if it is equal to 1. Previously the code was calling drop() but that drops all dimensions equal to 1. @andrjohns I'd appreciate if you can check my logic here, but I think with a subset of draws we want to drop only the first dimension if it is equal to 1. This passes all the tests I have setup for weird dimension sized parameter types so I think this logic is correct but I'd appreciate another set of eyes.

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@mhollanders would you mind installing the branch of cmdstanr and checking whether this works for the larger model you are working with? I have this passing your minimal test case but would rather be certain I'm not just catching a small edge case

[andrjohns]

Thanks! I'll sort out the windows tests and review tomorrow

[andrjohns]

@SteveBronder for some reason the test model only had a single unique lw value at this step in process_init_approx when psis_resample=TRUE - but only on Windows, and only when called in testthat::expect_no_error() (which is all kinds of bizarre)

I've set the test to use psis_resample=FALSE for now, but it might suggest there's some kind of weird interaction/issue with the process on Windows

[andrjohns]

LGTM thanks!

[mhollanders]

@mhollanders would you mind installing the branch of cmdstanr and checking whether this works for the larger model you are working with? I have this passing your minimal test case but would rather be certain I'm not just catching a small edge case

I checked and it works with my model. Thanks!