Add missing DEFAULT_SIGMA in MMD unit tests

Author: jaime-cespedes-sisniega

frouros/tests/unit/detectors/data_drift/batch/distance_based/test_mmd.py

@@ -1,7 +1,11 @@
 """Test MMD."""
 
 from functools import partial
-from typing import Any, Optional, Tuple
+from typing import (
+    Any,
+    Optional,
+    Tuple,
+)
 
 import numpy as np
 import pytest
@@ -182,7 +186,10 @@ def test_mmd_chunk_size_initialization_valid(
     X_ref = np.random.normal(0, 1, 100)
     X_test = np.random.normal(0, 1, 100)
 
-    kernel = partial(rbf_kernel, sigma=DEFAULT_SIGMA)
+    kernel = partial(
+        rbf_kernel,
+        sigma=DEFAULT_SIGMA,
+    )
 
     detector = MMD(
         kernel=kernel,
@@ -213,7 +220,10 @@ def test_mmd_chunk_size_invalid(
     :param chunk_size: chunk size to test
     :type chunk_size: Any
     """
-    kernel = partial(rbf_kernel, sigma=0.5)
+    kernel = partial(
+        rbf_kernel,
+        sigma=DEFAULT_SIGMA,
+    )
 
     with pytest.raises((TypeError, ValueError)):
         MMD(