Update base for 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