Update return type of train_test_split

[JanSkn]

Is your feature request related to a problem? Please describe.
Return type of train_test_split in pylearn/model_utils.py is incorrect.

Describe the solution you'd like
Current code:

def train_test_split(X: np.ndarray, y: np.ndarray, test_size=0.2, shuffle=True, random_seed=None) -> np.ndarray:
    ...

    return np.array(X_train), np.array(X_test), np.array(y_train), np.array(y_test)

Correct return type should be tuple of numpy arrays

Describe alternatives you've considered
/

Additional context
Would be version patch: 1.2.0 --> 1.2.1