Add docstrings to test class and methods

Author: HeyangQin

tests/transforms/test_rand_grid_distortiond.py

@@ -77,8 +77,11 @@
 
 
 class TestRandGridDistortiond(unittest.TestCase):
+    """Test cases for RandGridDistortiond dictionary transform."""
+
     @parameterized.expand(TESTS)
     def test_rand_grid_distortiond(self, input_param, seed, input_data, expected_val_img, expected_val_mask):
+        """Verify distortion produces expected output for image and mask keys."""
         g = RandGridDistortiond(**input_param)
         g.set_random_state(seed=seed)
         result = g(input_data)