[torchgen] Rename executorch's RuntimeContext to KernelRuntimeContext (#104892)
Rename the context type to match changes in executorch.
Differential Revision: [D46977359](https://our.internmc.facebook.com/intern/diff/D46977359/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/104892
Approved by: https://github.com/larryliu0820
