Reapply "Add (and fix) OSS check that et_operator_library dtype_selective_build builds (#9245)"

This was reverted due to internal CI failure; reapplying with
that fixed (the new Buck target needed to be moved).

Original summary: This wasn't checked, and it's easy to forget to update the all_deps
targets if we don't check. Now we check.

Differential Revision: [D76458040](https://our.internmc.facebook.com/intern/diff/D76458040/)

ghstack-source-id: 289775426
Pull Request resolved: #11573

Author: swolchok

.ci/scripts/unittest-buck2.sh

@@ -25,7 +25,9 @@ BUILDABLE_KERNELS_PRIM_OPS_TARGETS=$(buck2 query //kernels/prim_ops/... | grep -
 # //runtime/kernel/... is failing because //third-party:torchgen_files's shell script can't find python on PATH.
 # //runtime/test/... requires Python torch, which we don't have in our OSS buck setup.
 for op in "build" "test"; do
-    buck2 $op $BUILDABLE_OPTIMIZED_OPS //kernels/portable/... \
+    buck2 $op $BUILDABLE_OPTIMIZED_OPS \
+          //examples/selective_build:select_all_dtype_selective_lib_portable_lib \
+          //kernels/portable/... \
           $BUILDABLE_KERNELS_PRIM_OPS_TARGETS //runtime/backend/... //runtime/core/... \
           //runtime/executor: //runtime/kernel/... //runtime/platform/...
 done

shim_et/xplat/executorch/codegen/codegen.bzl

@@ -752,7 +752,7 @@ def executorch_generated_lib(
             See: https://www.internalfb.com/wiki/PyTorch/Teams/Edge/PyTorch_Edge_Core_Team/Dtype_Selective_Build/""")
 
     if dtype_selective_build:
-        if not expose_operator_symbols and not is_xplat():
+        if not expose_operator_symbols and not (is_xplat() or runtime.is_oss):
             fail("""
                 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