[ML] Fix cause of SIGSEGV for classification and regression #1379
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We weren't accounting for the padding we include since we memory aligned columns, #1142, in
CBoostedTreeFactory
when computing the number of extra columns. This meant we were using presenting extra columns to feature selection (which would nearly always ignore them) and occasionally changing the initialisation of the learn rate.The problem we hit which caused the SIGSEGV was when there was only one feature which itself carried no information, i.e. it was a constant. We could then erroneously select an extra column as a feature.