Restore _init_weights value in no_init_weights
Fend for potential nested use, and match the intuitive expectation for context managers().
In addition, users might modify private no_init_weights as well.

@patrickvonplaten @stas00 
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger has also been contributing extensively to that part of the code and might have something to say!<|||||>Thanks a lot for iterating on this!