Fix incorrect RMSE calculation

[evenmn]

This PR fixes a bug in the RMSE metric implementation.

Previously, the RMSE function was computing the square of the mean difference between two arrays X and T, rather than the mean of the squared differences. This resulted in incorrect values being returned.

Buggy implementation:

def rmse(X, T):
    return np.sqrt(np.mean(X - T) ** 2)

Corrected implementation:

def rmse(X, T):
    return np.sqrt(np.mean((X - T) ** 2))

This change ensures that the RMSE is calculated according to its standard definition.