keras/src/metrics/reduction_metrics.py/Mean/__init__
class Mean:    def __init__(self, name="mean", dtype=None):
        super().__init__(name=name, dtype=dtype)
        self.total = self.add_variable(
            shape=(),
            initializer=initializers.Zeros(),
            dtype=self.dtype,
            name="total",
        )
        self.count = self.add_variable(
            shape=(),
            initializer=initializers.Zeros(),
            dtype=self.dtype,
            name="count",
        )