Add forward and backward support for silu to NestedTensors (#97181)
# Summary
Add forward and backward support for silu to NestedTensors
- Add forward support to silu
- Add forward support to silu_
- Add backward support to silu
- Add to NT docs
- Add tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/97181
Approved by: https://github.com/cpuhrsch, https://github.com/jbschlosser
