Add `TORCH_CPP_LOG_LEVEL` to the docs
Fixes #70667
`TORCH_CPP_LOG_LEVEL=INFO` is needed for `TORCH_DISTRIBUTED_DEBUG` to be effective.
For reference, https://github.com/pytorch/pytorch/pull/71746 introduced the environment variable `TORCH_CPP_LOG_LEVEL` and https://github.com/pytorch/pytorch/pull/73361 documented it.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/76625
Approved by: https://github.com/rohan-varma
