[New operator] SparseLengthsSumFused8BitRowwise

[artemrakhov-glow]

We need to support SparseLengthsSumFused8BitRowwise operator from Caffe2. Spec can be found here:
https://caffe2.ai/docs/operators-catalogue.html#sparselengthssumfused8bitrowwise

Also there is weighted version: SparseLengthsWeightedSumFused8BitRowwise
We already support float version: SparseLengthsSum / SparseLengthsWeightedSum

[rdzhabarov]

cc: @beicy ^

[artemrakhov-glow]

@beicy : any updates on this? It is the most important among our unimplemented ops. Feel free to unassign if its not on your radar.

[beicy]

@artemrakhov Sorry, forget that it is still assigned to me... Unsigned it for now.

[jfix71]

@artemrakhov Were you going to take this? Otherwise I can work on it.

[artemrakhov-glow]

No, I wasn't. Thanks for taking this, @jfix71