def __init__(self):
        """Creates an instance of the standalone R2 metric.

        By default this metric in its initial state will return an R2
        value of 0. The metric can be updated by using the `update` method
        while the running R2 can be retrieved using the `result` method.
        """
        self._mean_R2 = Mean()
        """The mean utility that will be used to store the running R2."""
Creates an instance of the standalone R2 metric.

        By default this metric in its initial state will return an R2
        value of 0. The metric can be updated by using the `update` method
        while the running R2 can be retrieved using the `result` method.
        
avalanche/evaluation/metrics/r2.py