Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35507
We want to split up the SparseLengthsSumSparse op into an indirection op and the SparseLengthsSum op so that we can lower the later part. The indirection part is a plain impl now.
Test Plan:
```
for i in `seq 10`; do buck test caffe2/caffe2/python/operator_test:lengths_reducer_fused_nbit_rowwise_ops_test -- test_sparse_lengths_sum_rowwise_sparse; done
```
Reviewed By: jspark1105
Differential Revision: D20683478
fbshipit-source-id: 509effe88719d20aa0c4783bbe0ce1f183ee473c
af4d86788c