fix: flatc fallback and skip custom-op test when test_lib missing

- _flatbuffer: when FLATC_EXECUTABLE is set but path does not exist (e.g.
  placeholder /path/to/flatc), fall back to 'flatc' on PATH so serialization
  tests pass without editing the environment.
- test_passes: skip test_to_out_variant_none_output when my_awesome_3rdparty_ns
  awesome_op is unavailable (test_lib not loaded under pytest).

Author: NefAI

exir/_serialize/_flatbuffer.py

@@ -197,6 +197,9 @@ def _run_flatc(args: Sequence[str]) -> None:
     else:
         # Expect the `flatc` tool to be on the system path or set as an env var.
         flatc_path = os.getenv("FLATC_EXECUTABLE")
+        if flatc_path and not os.path.isfile(flatc_path):
+            # Env set to a path that doesn't exist (e.g. placeholder); use PATH.
+            flatc_path = "flatc"
         if not flatc_path:
             flatc_path = "flatc"
         subprocess.run([flatc_path] + list(args), check=True)

exir/tests/test_passes.py

@@ -115,6 +115,15 @@ def collect_ops(gm: torch.fx.GraphModule):
     return ops
 
 
+def _has_awesome_op_out() -> bool:
+    """True if test_lib is loaded and my_awesome_3rdparty_ns.awesome_op is available."""
+    try:
+        getattr(torch.ops.my_awesome_3rdparty_ns, "awesome_op").out
+        return True
+    except AttributeError:
+        return False
+
+
 lib = Library("DO_NOT_USE_TEST_ONLY", "DEF")
 
 lib.define("foo(Tensor self) -> (Tensor, Tensor)")
@@ -351,6 +360,10 @@ def forward(self, x: torch.Tensor) -> torch.Tensor:
         # TODO(angelayi): Add a utility function that verifies a model is in
         # the edge dialect
 
+    @unittest.skipIf(
+        not _has_awesome_op_out(),
+        "test_lib not loaded (my_awesome_3rdparty_ns.awesome_op unavailable)",
+    )
     def test_to_out_variant_none_output(self) -> None:
         class CompositeModel(torch.nn.Module):
             def __init__(self, _weight):