Set `val` inside the optimizer dictionary at `key`.

def set_val(self, key:str, val:Any, bn_groups:bool=True)->Any:
        "Set `val` inside the optimizer dictionary at `key`."
        if is_tuple(val): val = [(v1,v2) for v1,v2 in zip(*val)]
        for v,pg1,pg2 in zip(val,self.opt.param_groups[::2],self.opt.param_groups[1::2]):
            pg1[key] = v
            if bn_groups: pg2[key] = v
        return val