Improve QNN backend AOT user experience (pytorch#18412)

Co-authored-by: Abhinay Kukkadapu <abhinayk@meta.com>

Author: pytorchbot

backends/qualcomm/__init__.py

@@ -1,13 +1,29 @@
 import os
 
-from .scripts.download_qnn_sdk import install_qnn_sdk, is_linux_x86
+from .scripts.download_qnn_sdk import install_qnn_sdk, is_linux_x86, QNN_ZIP_URL
 
 
 env_flag = os.getenv("EXECUTORCH_BUILDING_WHEEL", "0").lower()
 # If users have preinstalled QNN_SDK_ROOT, we will use it.
 qnn_sdk_root_flag = os.getenv("QNN_SDK_ROOT", None)
 
-if env_flag not in ("1", "true", "yes") and not qnn_sdk_root_flag and is_linux_x86():
-    ok = install_qnn_sdk()
-    if not ok:
-        raise RuntimeError("Failed to install QNN SDK. Please check the logs above.")
+if env_flag not in ("1", "true", "yes"):
+    if qnn_sdk_root_flag:
+        print(
+            f"[QNN] Using QNN SDK at {qnn_sdk_root_flag} (from QNN_SDK_ROOT)",
+            flush=True,
+        )
+    elif is_linux_x86():
+        ok = install_qnn_sdk()
+        if not ok:
+            raise RuntimeError(
+                "Failed to set up QNN SDK.\n\n"
+                "To resolve, try one of:\n"
+                "  1. Download the SDK manually from:\n"
+                f"       {QNN_ZIP_URL}\n"
+                "     Or go to step 2 if QNN SDK already exists.\n"
+                "  2. Set QNN_SDK_ROOT to an existing SDK installation:\n"
+                "       export QNN_SDK_ROOT=/path/to/qualcomm/sdk\n"
+                "       export LD_LIBRARY_PATH="
+                "$QNN_SDK_ROOT/lib/x86_64-linux-clang/:$LD_LIBRARY_PATH"
+            )