Revert D23519521: [dper3] replace LengthsGather lowlevel module's PT implemetnatio to use caffe2 op
Test Plan: revert-hammer
Differential Revision:
D23519521 (https://github.com/pytorch/pytorch/commit/8c64bb4f4740b7d4441b8538a517fb1440b321c4)
Original commit changeset: ed9bd16a8af3
fbshipit-source-id: 33631299eabec05a1a272bfd0040d96203cf62a0
