add eq.str, ne.str, and add.str ops (#40958)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40958
add 3 str operators to lite interpreter
eq.str
ne.str
add.str
Test Plan:
```
buck run //xplat/caffe2/fb/pytorch_predictor:converter /mnt/vol/gfsfblearner-altoona/flow/data/2020-06-29/1ca8a85f-dbd5-4181-b5fc-63d24465c1fc/201084299/2068673333/model.pt1 ~/model_f201084299.bc
buck run xplat/assistant/model_benchmark_tool/mobile/binary/:lite_predictor -- --model ~/model_f201084299.bc --input_file /tmp/gc_model_input.txt --model_input_args src_tokens,dict_feat,contextual_token_embedding --warmup 1 --iter 2
```
Reviewed By: pengtxiafb
Differential Revision: D22369579
fbshipit-source-id: 7ac9a184d437c875edfb584221edd706bffb16e1
