Adding support to offsets based Fused8BitRowwiseEmbeddingLookup (#27635)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27635
PyTorch uses `offsets` instead of `lengths` for embedding table lookup. Adding support to that for fused quantized version.
AVX2 version is generated with
```
python caffe2/caffe2/perfkernels/hp_emblookup_codegen.py --fused --use-offsets
```
Test Plan:
```
buck test caffe2/torch/fb/sparsenn:test
```
Reviewed By: jianyuh
Differential Revision: D17826873
fbshipit-source-id: 23c4a96d92521deaebc02b688ad735d76a4476df
