FIX: Allow staticmethod be passed to callable metadata in ouput spec

Author: ghisvail

pydra/engine/specs.py

@@ -593,7 +593,12 @@ def _field_metadata(
                     return attr.NOTHING
                 return val
         elif "callable" in fld.metadata:
-            call_args = inspect.getfullargspec(fld.metadata["callable"])
+            callable_ = fld.metadata["callable"]
+            if isinstance(callable_, staticmethod):
+                # In case callable is defined as a static method,
+                # retrieve the function wrapped in the descriptor.
+                callable_ = callable_.__func__
+            call_args = inspect.getfullargspec(callable_)
             call_args_val = {}
             for argnm in call_args.args:
                 if argnm == "field":
@@ -615,7 +620,7 @@ def _field_metadata(
                             f"has to be in inputs or be field or output_dir, "
                             f"but {argnm} is used"
                         )
-            return fld.metadata["callable"](**call_args_val)
+            return callable_(**call_args_val)
         else:
             raise Exception("(_field_metadata) is not a current valid metadata key.")