[JIT] Remove references to no longer generated _tanh_backward and _sigmoid_backward (#44138)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44138
If you look at the sigmoid and tanh backward they are composed of other ops: https://github.com/pytorch/pytorch/blob/master/torch/csrc/jit/runtime/symbolic_script.cpp#L786
https://github.com/pytorch/pytorch/blob/master/torch/csrc/jit/runtime/symbolic_script.cpp#L164
So tanh_backward and sigmoid_backward are no longer generated / legacy ops.
Test Plan: Imported from OSS
Reviewed By: ZolotukhinM
Differential Revision: D23543603
Pulled By: eellison
fbshipit-source-id: ce8353e53043cf969b536aac47c9576d66d4ce02
