Remove unboxedAutogradKernel from c10 (#26130)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26130
Since we now just use TensorTypeId::VariableTensorId, there's no need to treat autograd kernels any differently.
ghstack-source-id: 90130457
Test Plan: unit tests
Differential Revision: D17353873
fbshipit-source-id: d4468506a5366bc5e7429144b090b3e78af9de62
