Classification category imbalance focal loss

Original reference: https://www.cnblogs.com/king-lps/p/9497836.html

Understanding of setting alpha to 0.25

The category is unbalanced.
Setting alpha to 0.25 for negative samples can reduce the impact of positive samples, but

it means that negative samples have become simple samples after gamma adjustment, and their loss may be lowered.
Alpha is set to 0.25. Balanced strategy. (This is a bit of alchemy)