Add description for test

theano/scan_module/tests/test_scan.py

@@ -2698,6 +2698,11 @@ class T_Scan(unittest.TestCase):
         utt.assert_allclose(f_vals, f2_vals)
 
     def test_gpu_memory_usage(self):
+        # This test validates that the memory usage of the defined theano
+        # function is reasonnable when executed on the GPU. It checks for
+        # a bug in which one of scan's optimization was not applied which
+        # made the scan node compute large and unnecessary outputs which
+        # brought memory usage on the GPU to ~12G.
 
         # The test must be performed on the GPU
         from theano.sandbox import cuda