bifpn to Downsampling sometime raise NotImplementedError

[km1562]

hello, sometime the bifpn never fit the condition in both if and elif,

so I print the h, w and target_h, target_w, it show that, when h and target_h equal to one, it will directly to raise NotImplementedError(), so I add a new condition to the branch, may be it can fix it,

[km1562]

and I saw the #473 have same question

[Yuliang-Liu]

@km1562 Thanks! Can you open a PR for that?