Register nested matmul as an addition to CompositeImplicit (#82786)
The initial matmul_nested in [#81957](https://github.com/pytorch/pytorch/pull/81957) is imperfect:
* it is allowed now to register another kernel in addition to CompositeImplicit
* so we should do that, instead of the code smell is_nested()
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82786
Approved by: https://github.com/albanD
