范例01：
//不提供sample_weight，默认等权重
y_true := array(1,1,1,-1);
y_pred := array(1,-1,1,1);
return matthews_corrcoef(y_true,y_pred); //-0.3333
范例02：
//提供sample_weight
y_true := array(1,1,1,-1);
y_pred := array(1,-1,1,1);
sample_weight := array(2,2,1,1);
return matthews_corrcoef(y_true,y_pred,sample_weight);
//-0.3162