Introduce an environment variable to change c10 log level (#71746)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71746
This PR contains the following improvements:
- It exposes a new environment variable `TORCH_CPP_LOG_LEVEL` that enables users to set the log level of c10 logging facility (supports both GLOG and c10 loggers). Valid values are `INFO`, `WARNING`, `ERROR`, and `FATAL` or their numerical equivalents `0`, `1`, `2`, and `3`.
- It implements an `initLogging()` function and calls it as part of `torch._C` module import to ensure that the underlying logging facility is correctly initialized in Python.
With these changes a user can dynamically set the log level of c10 as in the following example:
```
$ TORCH_CPP_LOG_LEVEL=INFO python my_torch_script.py
```
ghstack-source-id: 149822703
Test Plan: Run existing tests.
Reviewed By: malfet
Differential Revision: D33756252
fbshipit-source-id: 7fd078c03a598595d992de0b474a23cec91838af
(cherry picked from commit 01d6ec6207faedf259ed1368730e9e197cb3e1c6)
