Allow kernel manual registration (#491)

Summary:
X-link: https://github.com/pytorch/pytorch/pull/110086

Pull Request resolved: https://github.com/pytorch/executorch/pull/491

Exposing a codegen mode for generating a hook for user to register their kernels.

If we pass `--manual-registration` flag to `gen_executorch.py`, we will generate the following files:
1. RegisterKernels.h which declares a `register_all_kernels()` API inside `torch::executor` namespace.
2. RegisterKernelsEverything.cpp which implements `register_all_kernels()` by defining an array of generated kernels.

This way user can depend on the library declared by `executorch_generated_lib` macro (with `manual_registration=True`) and be able to include `RegisterKernels.h`. Then they can manually call `register_all_kernels()` instead of relying on C++ static initialization mechanism which is not available in some embedded systems.

Reviewed By: cccclai

Differential Revision: D49439673

fbshipit-source-id: 50ee80e1040e5dd32eb94f08c78f1544811cd08d