multiline KeyError msg python bug workaround (#18557)
Summary:
make multiline KeyError msg readable by working around a python bug https://bugs.python.org/issue2651
discussion: https://github.com/pytorch/pytorch/issues/16647
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18557
Differential Revision: D14681086
Pulled By: soumith
fbshipit-source-id: acbd13a823302c854c3d364028ed414fd8ce6bc8
