add a test that reproduces a memory leak in learner (skipped state)

tests/test_text_train.py

@@ -51,3 +51,23 @@ def test_classifier():
             assert last_layer(classifier.model).out_features == n_labels if n_labels > 1 else n_labels+1
         finally:
             shutil.rmtree(path)
+
+# XXX: may be move into its own test module?
+import gc
+# everything created by this function should be freed at its exit
+def clean_destroy_block():
+    path, df_trn, df_val = prep_human_numbers()
+    data = TextLMDataBunch.from_df(path, df_trn, df_val, tokenizer=Tokenizer(BaseTokenizer))
+    learn = language_model_learner(data, emb_sz=100, nl=1, drop_mult=0.)
+    learn.lr_find()
+
+@pytest.mark.skip(reason="memory leak to be fixed")
+def test_mem_leak():
+    gc.collect()
+    garbage_before = len(gc.garbage)  # should be 0 already, or something leaked earlier
+    assert garbage_before == 0
+    clean_destroy_block()
+    gc_collected = gc.collect() # should be 0 too - !0 means we have circular references
+    assert gc_collected == 0
+    garbage_after = len(gc.garbage)  # again, should be 0, or == garbage_before
+    assert garbage_after == 0