Defaults or _defaults?

I noticed that in the definition of “adam_step”, “_defaults” used instead of “defaults”. This is also used for other two optimizers, RAdam and QHAdam. Who knows why “_defaults” used? it actually make the defaults number useless, although the default esp always provided in the Adam function.

def adam_step(p, lr, mom, step, sqr_mom, grad_avg, sqr_avg, eps, **kwargs):> 
    "Step for Adam with `lr` on `p`"> 
    debias1 = debias(mom,     1-mom,     step)> 
    debias2 = debias(sqr_mom, 1-sqr_mom, step)>, (sqr_avg/debias2).sqrt() + eps, value = -lr / debias1)> 
    return p> 
adam_step._defaults = dict(eps=1e-5)