Warn when kernel version is too low on Linux (#2077)
* Warn when kernel version is too low on Linux
See #1929
On Linux with kernel version < 5.5, issues with hanging processes have
been reported. It is not clear how to fix the issue, so instead we warn
the user that they may encounter problems.
Notes
As logging requires an initialized PartialState, the actual check
happens at the end of Accelerator.__init__.
In a similar vein, the docstring of get_logger has been adjusted to
first initialize the Accelerator, as it is not working as currently
shown.
* Reviewer comment: small change to docstring
