Use explicit Operator.MAX to aid compilation

Fix handling of explicit operators in `allreduce`

pde/solvers/explicit_mpi.py

@@ -121,12 +121,14 @@ def _make_error_synchronizer(self) -> Callable[[float], float]:
         """return helper function that synchronizes errors between multiple processes"""
         if mpi.parallel_run:
             # in a parallel run, we need to return the maximal error
-            from ..tools.mpi import mpi_allreduce
+            from ..tools.mpi import Operator, mpi_allreduce
+
+            operator_max_id = Operator.MAX
 
             @register_jitable
             def synchronize_errors(error: float) -> float:
                 """return maximal error accross all cores"""
-                return mpi_allreduce(error, "MAX")  # type: ignore
+                return mpi_allreduce(error, operator_max_id)  # type: ignore
 
             return synchronize_errors  # type: ignore
         else:

pde/tools/mpi.py

@@ -197,6 +197,8 @@ def ol_mpi_allreduce(data, operator: int | str | None = None):
         else:
             # assume an operator is specified by it's id
             op_id = int(operator.literal_value)
+    elif isinstance(operator, nb.types.Integer):
+        op_id = None  # use given value of operator
     else:
         raise RuntimeError(f"`operator` must be a literal type, not {operator}")
 
@@ -205,6 +207,8 @@ def _allreduce(sendobj, recvobj, operator: int | str | None = None) -> int:
         """helper function that calls `numba_mpi.allreduce`"""
         if operator is None:
             return numba_mpi.allreduce(sendobj, recvobj)  # type: ignore
+        elif op_id is None:
+            return numba_mpi.allreduce(sendobj, recvobj, operator)  # type: ignore
         else:
             return numba_mpi.allreduce(sendobj, recvobj, op_id)  # type: ignore