[pytorch] Route default warning sync to LOG(WARNING) (#36768)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/36768
Follow LOG(WARNING) format for c++ side warnings in order to play well with larger services, especially when using glog. I need to hook up into GLOG internals a bit in order to override FILE/LINE without having to change the whole thing to be macros, but it seems to be stable between glog versions.
Note, this also changes caffe2_log_level to warning by default - I think it's a much better default when compiling without glog (or maybe even have info)
Test Plan:
Run unittest in both glog and non-glog build mode:
glog:
```
W0416 12:06:49.778215 3311666 exception_test.cpp:23] Warning: I'm a warning (function TestBody)
```
no-glog:
```
[W exception_test.cpp:23] Warning: I'm a warning (function TestBody)
```
Reviewed By: ilia-cher
Differential Revision: D21078446
fbshipit-source-id: b5d36aac54d6b6295a72de6754696ccafbcb84ca
