[ML] Fix weights to maximize minimum recall for multiclass classification #1231
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This is a correction for #1113.
We were computing the objective on the full training set rather than the sampled mask. Normally this is fine, since we use a stratified sample it basically acts like a constant scaling of the objective. However, when there is very little training data we can run into problems (such as a class missing in the sample set which is present in the training set). It also means this code is much slower than intended on very large data sets.
Change #1113 hasn't been released so I've marked this as a non-issue, but it must go out at the same time in 7.8.