Fix labs effective_hamiltonian test (#8257)

**Context:**
A generator in effective_hamiltonian is updated to fix an issue raised
by concurrency for parallel runs with `mpi4py_pool`.

**Description of the Change:**

**Benefits:**

**Possible Drawbacks:**

**Related GitHub Issues:**

---------

Co-authored-by: Will <[email protected]>

Author: soranjh

doc/releases/changelog-dev.md

@@ -741,6 +741,7 @@
 
 * Added concurrency support for `effective_hamiltonian` in labs.
   [(#8081)](https://github.com/PennyLaneAI/pennylane/pull/8081)
+  [(#8257)](https://github.com/PennyLaneAI/pennylane/pull/8257)
 
 * Fixed a queueing issue in `ResourceOperator` tests.
   [(#8204)](https://github.com/PennyLaneAI/pennylane/pull/8204)

pennylane/labs/trotter_error/product_formulas/error.py

@@ -96,11 +96,11 @@ def effective_hamiltonian(
     with executor(max_workers=num_workers) as ex:
         partial_sum = ex.starmap(
             _eval_commutator,
-            (
+            [
                 (commutator, coeff, fragments)
                 for ith_order in bch
                 for commutator, coeff in ith_order.items()
-            ),
+            ],
         )
 
     eff = _AdditiveIdentity()