Dtype selective build: fail if not xplat, if portable/optimized not in kernel_deps

#10985

Try to make user error harder for dtype selective build.

Emit warning for now, as too many failures when set to failure :(
For example:
```
buck2 build //xplat/sgr/resources/tests/handwriting:pkg
buck2 build fbsource//xplat/sgr/resources/mwa:main_pkg_libAndroid

Differential Revision: [D75027794](https://our.internmc.facebook.com/intern/diff/D75027794/)

[ghstack-poisoned]

Author: lucylq

shim_et/xplat/executorch/codegen/codegen.bzl

@@ -641,6 +641,26 @@ def executorch_generated_lib(
     if not aten_mode and not functions_yaml_target and not custom_ops_yaml_target:
         fail("At least one of functions_yaml_target, custom_ops_yaml_target needs to be provided")
 
+    if dtype_selective_build:
+        if not is_xplat():
+            warning("""
+            !!WARNING!! Dtype_selective_build is only available in xplat.
+            Proceeding without dtype selective build for lib: {}.""".format(name))
+        
+        # Dtype selective build is enabled on portable and optimized kernel libraries. 
+        if (not "//executorch/kernels/portable:operators" in kernel_deps) and (not "//executorch/kernels/optimized:optimized_operators" in kernel_deps):
+            warning("""
+            !!WARNING!! Dtype_selective_build is enabled but kernel_deps does not contain portable or optimized dependencies.
+            Please add one of these to kernel_deps in `executorch_generated_lib`:
+            //executorch/kernels/portable:operators
+            //executorch/kernels/optimized:optimized_operators
+            This will tell the build system to rebuild portable/optimized with dtype selective build enabled.
+            For examples, see: //executorch/examples/selective_build/targets.bzl
+            Currently, kernel_deps contains {}.
+
+            Proceeding without dtype selective build.
+            """.format(kernel_deps))
+
     aten_suffix = "_aten" if aten_mode else ""
 
     # merge functions.yaml with fallback yaml