Fuse to individual operators to GatherFuse8BitRowwiseQuantFloatMulLengthElim (#25519)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25519
Fuse Gather-Fused8BitRowwiseQuantizedToFloat-Mul-LengthsSum opportunistically.
Test Plan:
```
buck test caffe2/caffe2/opt/custom:concat_elim_test
```
Reviewed By: dreamingleo
Differential Revision: D17125045
fbshipit-source-id: 8ee50410eb13a82e1e5c8180f392fce2fe9cd728
