dtype selective build, link codegen and scalar_type_util (#1090)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1090

1. Let codegen generate `selected_mobile_ops.h` based on `selected_operators.yaml`
2. Include header in `scalar_type_utils`. Add to visibility for codegen.
3. Create test for dtype selective build in scalar_utils.
4. Add buck config for dtype selective build.

Dependencies:
1. Make scalar_type_utils depend on codegen headers (header lib only) to access `selected_mobile_ops.h`.
2. Split deps into `deps` and `kernel_deps`. Add  `kernel_deps` to executorch_generated_library. There is a circular dependency if we leave it as is, where:
- scalar_type_utils -> executorch_generated_lib (added dep)
- executorch_generated_lib -> portable_ops (portable ops are a dependency to the genrule for `selected_operators.yaml`, but they do not need to be)
- portable_ops -> kernel impls (eg. `op_add.cpp` etc.)
- kernel impls -> scalar_type_utils (for macros)

User flow:
1. User passes in the required executorch_generated_lib through buck config
2. See example in selective build examples.

To do: include another dtype selection example when we can use model.pte for selective build.

Reviewed By: larryliu0820

Differential Revision: D50561135

fbshipit-source-id: c52b6318bc1909c9c73a0ba231ecc2a6f01d1d22