Port silu_backward to structured (#58661)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58661
I removed dispatch: CompositeImplicitAutograd: math_silu_backward
Definitely not right, but I don't know how it works with structured core.
Keeping it will trigger an assertion failure
```
assert dispatch.keys() != {DispatchKey.CompositeImplicitAutograd}, \
f"unexpected name for singleton CompositeImplicitAutograd dispatch entry: expected {cpp.name(func)} " \
f"but got {dispatch[DispatchKey.CompositeImplicitAutograd]}. Rename your implementation to the expected " \
"name, then delete the dispatch table"
```
Test Plan: Imported from OSS
Reviewed By: soulitzer
Differential Revision: D28572530
Pulled By: ezyang
fbshipit-source-id: 410f03bddf79cda7c9f0fd66f697383ee2925d32
