Fix inv_metric()

[rok-cesnovar]

Summary

Fixes #393

Copyright and Licensing

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Rok Češnovar

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)

[rok-cesnovar]

Also ran the example from @maxmantei and with this patch, it runs for all iterations.

library(cmdstanr)
for (i in 1:100){
  f <- cmdstanr_example(method = "sample", metric = "dense_e")
  M <- f$inv_metric()
  if (length(M) != 4){
    print(paste("Try:", i))
    print(M)
    break
  }
}

[codecov-io]

Codecov Report

Merging #394 (bd74210) into master (075a595) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #394      +/-   ##
==========================================
+ Coverage   88.34%   88.37%   +0.02%     
==========================================
  Files          12       12              
  Lines        2720     2726       +6     
==========================================
+ Hits         2403     2409       +6     
  Misses        317      317              

Impacted Files	Coverage Δ
R/read_csv.R	98.80% <100.00%> (+0.02%)	⬆️

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[jgabry]

Looks good! The tests are all starting again because I made an edit to the NEWS file but they were all passing so I'm going to go ahead and merge this.