Optionally add qnn backend to llama runner buck file

Summary: Include qnn backend as part of the llama runner dependency, and it's control by a build flag. Default to false.

Differential Revision: D64334713

Author: cccclai

examples/models/llama/runner/targets.bzl

@@ -12,7 +12,6 @@ def _get_operator_lib(aten = False):
 def define_common_targets():
     for aten in (True, False):
         aten_suffix = "_aten" if aten else ""
-
         runtime.cxx_library(
             name = "runner" + aten_suffix,
             srcs = [
@@ -27,7 +26,6 @@ def define_common_targets():
             visibility = [
                 "@EXECUTORCH_CLIENTS",
             ],
-            # qnn_executorch_backend can be added below //executorch/backends/qualcomm:qnn_executorch_backend
             exported_deps = [
                 "//executorch/backends/xnnpack:xnnpack_backend",
                 "//executorch/extension/llm/runner:stats",
@@ -46,7 +44,15 @@ def define_common_targets():
                 # Vulkan API currently cannot build on some platforms (e.g. Apple, FBCODE)
                 # Therefore enable it explicitly for now to avoid failing tests
                 "//executorch/backends/vulkan:vulkan_backend_lib",
-            ] if native.read_config("llama", "use_vulkan", "0") == "1" else []),
+            ] if native.read_config("llama", "use_vulkan", "0") == "1" else []) + ([
+                # //executorch/backends/qualcomm:qnn_executorch_backend doesn't work, 
+                #  likely due to it's an empty library with dependency only
+                "//executorch/backends/qualcomm/runtime:runtime",
+            ] if native.read_config(
+                "executorch",
+                "enable_qnn",
+                "false",
+            ) else []),
             external_deps = [
                 "libtorch",
             ] if aten else [],