keras/src/constraints/constraints.py/Constraint/__call__
class Constraint:    def __call__(self, w):
        """Applies the constraint to the input weight variable.

        By default, the inputs weight variable is not modified.
        Users should override this method to implement their own projection
        function.

        Args:
            w: Input weight variable.

        Returns:
            Projected variable (by default, returns unmodified inputs).
        """
        return w