Fix: Compare input against beta * threshold in softplus backwards (#56484)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/55587
The fix converts the binary `TensorIterator` used by softplus backwards to a ternary one, adding in the original input for comparison against `beta * threshold`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56484
Reviewed By: malfet
Differential Revision: D27908372
Pulled By: jbschlosser
fbshipit-source-id: 73323880a5672e0242879690514a17886cbc29cd
