Don't log an error when there are no CUDA backend symbols to promote (#23895)

### Summary

`load_library` re-opens the library that holds the CUDA backend with
`RTLD_NOLOAD | RTLD_GLOBAL` to make its symbols global. When the backend
is linked into the executable, as in a statically linked runner,
`dladdr` names the executable, and `dlopen` finds no library to re-open
and sets no error. So every first load logged "Failed to promote
symbols: (null)", although the executable's symbols are already global
and there is nothing to promote. The error is now logged only when
`dlopen` reports one; `dlerror()` is cleared before the call so a stale
error is not reported.

This PR was authored with AI (Claude Code).

### Test plan

`backends/cuda/runtime/platform/platform.cpp` compiles (`g++ -std=c++17
-fsyntax-only -Wall -Wextra`). The `dlopen` behavior the change relies
on, checked with a small program that calls `dlopen(<its own path>,
RTLD_NOW | RTLD_GLOBAL | RTLD_NOLOAD)` on glibc 2.39 (Jetson AGX Thor,
Ubuntu 24.04):

```
dli_fname=./noload handle=(nil) dlerror=(null)
```


cc @cccclai @nil-is-all @Gasoonjia @digantdesai

Author: edmundshieh

backends/cuda/runtime/platform/platform.cpp

@@ -86,11 +86,24 @@ executorch::runtime::Result<void*> load_library(
     Dl_info info;
     // Get info about a symbol we know exists in this backend library
     if (dladdr((void*)&load_library, &info) && info.dli_fname) {
-      // Re-open with RTLD_GLOBAL | RTLD_NOLOAD to promote symbols
+      // Re-open with RTLD_GLOBAL | RTLD_NOLOAD to promote symbols. Clear any
+      // earlier error first, so dlerror() below reports only this call's.
+      dlerror();
       void* handle =
           dlopen(info.dli_fname, RTLD_NOW | RTLD_GLOBAL | RTLD_NOLOAD);
       if (!handle) {
-        ET_LOG(Error, "Failed to promote symbols: %s", dlerror());
+        const char* error = dlerror();
+        if (error != nullptr) {
+          ET_LOG(Error, "Failed to promote symbols: %s", error);
+        } else {
+          // dlopen found no library to re-open and set no error. That is the
+          // case when this backend is linked into the executable, whose
+          // symbols are global already.
+          ET_LOG(
+              Debug,
+              "Not promoting symbols: %s is not loaded as a shared library",
+              info.dli_fname);
+        }
       } else {
         // Close the handle after successful promotion
         dlclose(handle);