[Backend Tester] Mark adaptive avgpool2d as an unsupported portable op (pytorch#13398)

It's causing failures for a few model tests. This change prevents these
failures from showing up as backend failures.

Author: GregoryComer

backends/test/suite/runner.py

@@ -15,6 +15,7 @@
 # Set of unsupported ops that should cause tests to be skipped
 UNSUPPORTED_PORTABLE_OPS = {
     "aten::_embedding_bag",
+    "aten::_adaptive_avg_pool2d",
     "aten::median",
     "aten::median.dim",
     "aten::round.decimals",