Replace non-printable-ascii characters in ProtoDebugString (#14918)

Replace non-printable-ascii characters in ProtoDebugString (#14918)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14918

When ProtoBuf-Lite is in use, ProtoDebugString just calls SerializeAsString.
This produces binary output, which is not a very suitable "debug" string.
Specifically, we've observed it causing problems when calling code tries to
add the debug string to a Java exception message (which requires valid UTF-8).
Now, we replace all non-ASCII bytes with "?".

This is not a very fast implementation, but generating debug strings shouldn't
be a performance-sensitive operation in any application.

Reviewed By: dzhulgakov

Differential Revision: D13385540

fbshipit-source-id: 8868172baf20efaf53fecf7d666a6980f59b64f5