Silence a CodeQL warning about overwritten value in tests.

Signed-off-by: Michal Zientkiewicz <michalz@nvidia.com>

dali/test/python/test_pipeline.py

@@ -2293,7 +2293,7 @@ def pdef(to_cpu):
  return to_cpu(gpu)
 
  with assert_raises(RuntimeError, glob="doesn't support transition from GPU to CPU"):
- pipe = pdef(lambda gpu: gpu.cpu()) # this will raise an error at construction time
+ _ = pdef(lambda gpu: gpu.cpu()) # this will raise an error at construction time
 
  pipe = pdef(lambda gpu: gpu._to_backend("cpu")) # this will not raise errors until build-time