Add template based unboxing (#1284)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1284

Adding a new feature to allow users to bypass codegen and register their
kernels directly. This is very useful for custom kernels for custom ops.

Example usage:
```
Tensor& my_op(RuntimeContext& ctx, const Tensor& self, const Tensor& other, Tensor& out) {
    // ...
    return out;
}

Kernel my_kernel = Kernel.make_boxed_kernel("my_ns::my_op",EXECUTORCH_FN(my_op));

register_kernels({my_kernel});
```

imported-using-ghimport

Test Plan: Imported from OSS

Reviewed By: iseeyuan

Differential Revision: D51553099

Pulled By: larryliu0820

fbshipit-source-id: 0e3877a481d58c4e64c7767f7693537407ab27c5