Add missing LTC headers, re-enble xla configuration
Addresses XLA test failures due to missing PyTorch lazy tensor backend headers:
```
“fatal error: ‘torch/csrc/lazy/backend/backend_device.h’ file not found” from pytorch-xla-linux-bionic-py3.7-clang8
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/74756
Approved by: https://github.com/seemethere
