reduce duplicate computation in poisson, gamma, and tweedie objectives (#4950)
reduce duplicate computation in poisson, gamma, and tweedie objectives (#4950)

* ENH save computations of exp in objectives

* CLN missing declaration
microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,henry0312/LightGBM,microsoft/LightGBM