fix: Add environment variable defaults to BACKENDS (#8465)

Co-authored-by: Dmitry Tokarev <dtokarev@nvidia.com>

Author: mc-nv

qa/L0_shared_memory/shared_memory_test.py

@@ -301,7 +301,13 @@ def test_too_big_shm(self):
 
     def test_large_shm_register_offset(self):
         # Test for out of bounds read vulnerability when registering system shared memory with large offset
-        for platform in ["python", "onnx", "libtorch", "plan", "openvino"]:
+
+        platforms = (
+            ["python", "onnx", "libtorch", "plan", "openvino"]
+            if os.environ.get("BACKENDS") is None
+            else os.environ.get("BACKENDS").split()
+        )
+        for platform in platforms:
             model_name = f"{platform}_int32_int32_int32"
 
             # Test for large offset

qa/L0_shared_memory/test.sh

@@ -160,7 +160,7 @@ done
 # Test large system shared memory offset
 rm -rf models/*
 # prepare add_sub model of various backends
-BACKENDS="python onnx libtorch plan openvino"
+BACKENDS=${BACKENDS:-"python onnx libtorch plan openvino"}
 for backend in ${BACKENDS} ; do
     model="${backend}_int32_int32_int32"
     model_dir="models/${model}"