Update coco.py

[JavierClearImageAI]

attempt to debug "getImgIds" for multiple categories ids

[nghiahhnguyen]

I welcome this change. The original implementation does not get all the corresponding images.

[nghiahhnguyen]

I think the reason they use &= is that the authors want getImgIds to get the intersection of classes (i.e., each retrieved image must have all the categories) while what we want is the union of classes (i.e., each retrieved image must have at least one category). I think it's more of a misunderstanding than a bug. And I hope that your fix can be a new function instead of modifying this function.

Someone explained it in this issue: #114 (comment)