Update on "[kernel] Add template based unboxing"

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});
```

Differential Revision: [D51553099](https://our.internmc.facebook.com/intern/diff/D51553099)

[ghstack-poisoned]

Author: larryliu0820

extension/kernel_util/meta_programming.h

@@ -5,6 +5,8 @@
  * This source code is licensed under the BSD-style license found in the
  * LICENSE file in the root directory of this source tree.
  */
+
+
 #pragma once
 #if __cplusplus < 201703L
 #error "This header requires C++17"

extension/kernel_util/test/targets.bzl

@@ -1,5 +1,4 @@
 load("@fbsource//xplat/executorch/build:runtime_wrapper.bzl", "runtime")
-load("@fbsource//xplat/executorch/codegen:codegen.bzl", "et_operator_library", "executorch_generated_lib")
 
 def define_common_targets():
     """Defines targets that should be shared between fbcode and xplat.