Dtype selective build refactor (#3938)

Summary:
Pull Request resolved: #3938

1. Add dtype_selective_build as an option to executorch_generated_lib
2. Copy portable source and header files into codegen.bzl directory (this is the caller's directory, eg. examples/selective_build)
3. If kernel_deps contains portable_ops, remove it and use the source files

Note: dtype selective build only available for xplat. `header_namespace` is only available in xplat, used to link headers into correct place.
```
error: fail: cpp_library(): the `header_namespace` parameter is *not* supported in fbcode -- `#include` paths must match their fbcode-relative path. (executorch/backends/vulkan/serialization/schema/vk_delegate_schema)
```

New API usage:

```
executorch_generated_lib(
...
dtype_selective_build = True,
...
)
```

Reviewed By: larryliu0820

Differential Revision: D58396325

fbshipit-source-id: 739ebc6b94ffa5f791e125442804a2fff27eb176

Author: lucylq

examples/selective_build/README.md

@@ -14,14 +14,15 @@ bash examples/selective_build/test_selective_build.sh [cmake|buck2]
 
 ## BUCK2 examples
 
-Check out `targets.bzl` for demo of 3 selective build APIs:
+Check out `targets.bzl` for demo of 4 selective build APIs:
 1. `--config executorch.select_ops=all`: Select all ops from the dependency kernel libraries, register all of them into ExecuTorch runtime.
 2. `--config executorch.select_ops=list`: Only select ops from `ops` kwarg in `et_operator_library` macro.
 3. `--config executorch.select_ops=yaml`: Only select from a yaml file from `ops_schema_yaml_target` kwarg in `et_operator_library` macro.
+4. `--config executorch.select_ops=dict`: Only select ops with from `ops_dict` kwarg in `et_operator_library` macro. Optionally, add dtype information to each operator and add `dtype_selective_build = True` to only select those dtypes.
+Eg. if the model only uses the float implementation of add, then only the float add will be registered. Note: setting `dtype_selective_build = True` is only available in xplat.
 
 Other configs:
 - `--config executorch.max_kernel_num=N`: Only allocate memory for the required number of operators. Take this result from `selected_operators.yaml`.
-- `--config executorch.dtype_selective_build_lib=<executorch_generated_lib_name>`: Use dtype selective build. For each op, we register the dtypes that are used. Eg. if the model only uses the float implementation of add, then only the float add will be registered. Pass in the executorch_generated_lib name (see buck2 model example in targets.bzl).
 
 ## CMake examples
 

examples/selective_build/targets.bzl

@@ -63,6 +63,7 @@ def define_common_targets():
         deps = [
             ":select_ops_in_dict",
         ],
+        dtype_selective_build = True,
         visibility = ["//executorch/..."],
     )