[fix] fix GatherInfoForThresholdNumerical boundary (fix #4286)

[shiyu1994]

This is to fix #4286. In GatherInfoForThresholdNumerical , since it is calculating the right sum statistics, it should break when static_cast<uint32_t>(t + offset) <= threshold instead of static_cast<uint32_t>(t + offset) < threshold. Because data with bin value <= threshold are split into the left child node.

[jameslamb]

nice, thank you!

[shiyu1994]

BTW, currently our force splits method only produces splits with default_left=true, so only sweeping from the right side of the histogram is implemented in the GatherInfoForThresholdNumerical. Meanwhile, for categorical feature, the split condition can only be a single category value, instead of a set of category values as in normal categorical splits. So I think it requires further efforts to complete the functionality of force splits.

[shiyu1994]

It seems that forced splits functionality is not covered by any test cases. I think we may need to add tests for it.

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