(fix) Change default == value to is

as the default can be an array. If that is the case the previous
implementation fails in the if clause as the truth value of an
array is ambiguous.

src/nnbench/runner.py

@@ -106,7 +106,7 @@ def _issubtype(t1: type, t2: type) -> bool:
         for bm in self.benchmarks:
             for var in bm.interface.variables:
                 name, typ, default = var
-                if default == empty:
+                if default is empty:
                     required.add(name)
                 if name in params and default != empty:
                     logger.debug(