lr - base_lr).
    The lr at any cycle is the sum of base_lr
    and some scaling of the amplitude; therefore
    max_lr may not actually be reached depending on
    scaling function.
step_size: number of training iterations per
    half cycle. Authors suggest setting step_size
    2-8 x training iterations in epoch.
mode: one of {triangular, triangular2, exp_range}.
    Default 'triangular'.
    Values correspond to policies detailed above.
    If scale_fn is not None, this argument is ignored.
gamma: cons