Quick fix for #18215, the CPU case (#21910)
Summary:
The bug is that when target_length == 0, there is no preceding BLANK state and the original implementation will lead to out of bound pointer access.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21910
Differential Revision: D15960239
Pulled By: ezyang
fbshipit-source-id: 7bbbecb7bf91842735c14265612c7e5049c4d9b3
