Have getFilesToLevels return a reference (#71047)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71047
The copy induced by getFilesToLevels is currently consuming 3,457,470,000 cycles per day. A reference might fix that.
Reference:
```
["Inline torch::jit::JitLoggingConfig::getFilesToLevels[abi:cxx11] @ caffe2/torch/csrc/jit/jit_log.cpp:54"]
```
Test Plan: Sandcastle
Reviewed By: ngimel
Differential Revision: D33479180
fbshipit-source-id: 05d306ad9ea23e2f30348a08d547ebe274eb0c10
