Set beta (or alpha as makes sense for given optimizer).

def beta(self, val:float)->None:
        "Set beta (or alpha as makes sense for given optimizer)."
        if val is None: return
        if 'betas' in self.opt_keys:    self.set_val('betas', (self._mom, listify(val, self._beta)))
        elif 'alpha' in self.opt_keys:  self.set_val('alpha', listify(val, self._beta))
        self._beta = listify(val, self._beta)