*It computes Normalized Discounted Cumulative Gain.
*This function sums up the true scores ranked in the order induced by the predicted scores, after applying a logarithmic discount.
*The following step is to divide by the best possible score to obtain a score between 0 and 1.
*It yields a high value if true labels are ranked high by y_score.
*y_true : array like structure
*y_score : array like structure
*Normalized discounted cumulative gain - a float value.
0.7776802164125916
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