fix problem

hysop/problem.py

 import datetime
 import sys
-from hysop.constants import Backend, MemoryOrdering
+from hysop.constants import Backend, MemoryOrdering, HYSOP_DEFAULT_TASK_ID
 from hysop.tools.string_utils import vprint_banner
 from hysop.tools.contexts import Timer
 from hysop.tools.decorators import debug
@@ -30,7 +30,8 @@ class Problem(ComputationalGraph):
                                     for node in ops if hasattr(node, 'impl_kwds') and 'variables' in node.impl_kwds
                                     for f in node.impl_kwds['variables'].keys()])
         if given_ops_have_tasks:
-            if len(set(ops_tasks)) == 1 and ops_tasks[0] == self.mpi_params.task_id:
+            pb_task_id = HYSOP_DEFAULT_TASK_ID if self.mpi_params is None else self.mpi_params.task_id
+            if len(set(ops_tasks)) == 1 and ops_tasks[0] == pb_task_id:
                 # Intertask is not needed this is a single task-problem
                 given_ops_have_tasks = False