graph mode: add handling for layer_norm op (#37525)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37525
Adds graph mode handling for the layer_norm op
Test Plan:
```
python test/test_quantization.py TestQuantizeScriptPTSQOps.test_layer_norm
```
Imported from OSS
Differential Revision: D21310360
fbshipit-source-id: 83f475fb30b89c29623b79a6a6bfd5c20c569b51
