Update on "Dtype selective build: enable in fbcode"

^
By removing `header_namespace` and depending on #include "selected_op_variants.h" instead of the full path #include <executorch/kernels/portable/cpu/selected_op_variants.h>

Note:
expose_operator_symbols=False only works in xplat, so add a failure message for that too. I don't think we should recommend for users to set it to true, as it prevents a library from linking multiple executorch_generated_libs (symbols will clash).

Differential Revision: [D75082395](https://our.internmc.facebook.com/intern/diff/D75082395/)

[ghstack-poisoned]

Author: lucylq

shim_et/xplat/executorch/codegen/codegen.bzl

@@ -644,6 +644,7 @@ def executorch_generated_lib(
 
     if dtype_selective_build:
         if not expose_operator_symbols and not is_xplat():
+            # TODO(T225169282): make this a fail once internal cases move to xplat.
             warning("""
                 Dtype selective build with expose_operator_symbols=False works only in xplat - 
                 there are undefined symbols otherwise. Please try to use xplat, or talk to the