Add reduction string test for ctc_loss. (#43884)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/43884
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D23427907
Pulled By: gchanan
fbshipit-source-id: 889bd92e9d3e0528b57e3952fc83e25bc7abe293
