Reserve vectors, to save reallocation costs. (#1949)
Reserve vectors, to save reallocation costs. (#1949)

File: [LightGBM//src/io/dataset.cpp]
Function: [138:FastFeatureBundling(...)]

Reserving vectors where we already know the size to save on reallocation costs.

Also removed a variable that was unnecessary.
microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM,microsoft/LightGBM,henry0312/LightGBM