FastTree learning rate in advanced settings does not override the default value

[yaeldekel]

These lines in the BoostingFastTreeTrainerBase ctor reset it to the value specified directly (or the default value):

        if (Args.LearningRates != learningRate)
        {
            using (var ch = Host.Start($"Setting learning rate to: {learningRate} as supplied in the direct arguments."))
                Args.LearningRates = learningRate;
        }

(Reported in #1983 .)