Transform dictionary from (key_a -> key_b -> float) to
    (key_a, key_b) -> float

def uncurry_nested_dictionary(curried_dict):
    """
    Transform dictionary from (key_a -> key_b -> float) to
    (key_a, key_b) -> float
    """
    result = {}
    for a, a_dict in curried_dict.items():
        for b, value in a_dict.items():
            result[(a, b)] = value
    return result