Mean regressor baseline

The problem, from first principles

The regression version of the majority-class baseline predicts the same value for every example. The best constant under squared error is the training mean. A regression model should beat it, and R-squared score is defined so that this baseline scores exactly zero.

Implement mean_regressor_baseline(y_train, n_samples), which returns an array of length n_samples filled with the mean of y_train as a float.

Constraints

• y_train must not be empty: raise ValueError.
• The output dtype is float, even if y_train holds integers.
• Do not modify y_train.

Hints

Open one at a time. Each gives away a little more than the last.