Revert D23557576: Revert D23519521: [dper3] replace LengthsGather lowlevel module's PT implemetnatio to use caffe2 op
Test Plan: revert-hammer
Differential Revision:
D23557576
Original commit changeset: 33631299eabe
fbshipit-source-id: 704d36a16346f047b30e2da8be882062135f8617
