Fix inconsistent anchor boxes generation between train and inference

* Currently, `AnchorGenerator` and `Anchor` can actually generate different anchor boxes when input image size is not divisible by 2^max_level. This results in inconsistent training and inference box predictions. This change fix the inconsistency by calling only the Anchor class.
* Fix the case when input image is not square.
* Refactor the Anchor class to only generate and store `multilevel_boxes` since the flatten boxes are never used.

PiperOrigin-RevId: 616997347