Enable splitSparseLengthsSumSparse in onnxifi (#35555)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35555
Att. So that we can lower the SparseLengthsSum* part of SparseLengthsSum*Sparse. We update the tying policy between Gather and SparsLengthsWeightSum* so that we don't bother lowering a single Gather into the backend, which is inefficient to execute on card and creates bubbles between continuous lowering graphs.
Test Plan:
```
buck test glow/fb/test:test_onnxifinnpi
```
Reviewed By: ipiszy
Differential Revision: D20688525
fbshipit-source-id: cb8e38239057ff13a8d385ed09d0d019421de78b
