Use python type string for user facing error msgs (#20657)
Summary:
Otherwise users see something like (Tensor, Tensor)? and don't know what the ? means.
First commit is formatting.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20657
Differential Revision: D15400225
Pulled By: eellison
fbshipit-source-id: cf826790bf2ddafd34f6d5c144526cad9904770b
