Retrieve last logged val from result by key (#3049)

* return last logged value

* Update test_results.py

* Update step_result.py

* Update step_result.py

* pep8

* pep8

pytorch_lightning/core/step_result.py

@@ -45,6 +45,12 @@ class Result(Dict):
             }
         }
 
+    def __getitem__(self, key: Union[str, Any]) -> Any:
+        try:
+            return super().__getitem__(key)
+        except KeyError:
+            return super().__getitem__(f'step_{key}')
+
     def __getattr__(self, key: str) -> Any:
         try:
             if key == 'callback_metrics':

tests/core/test_results.py

@@ -236,3 +236,11 @@ def test_result_gather_mixed_types():
     expected = [1.2, ["bar", None], torch.tensor(1)]
     assert isinstance(result["foo"], list)
     assert result["foo"] == expected
+
+
+def test_result_retrieve_last_logged_item():
+    result = Result()
+    result.log('a', 5., on_step=True, on_epoch=True)
+    assert result['epoch_a'] == 5.
+    assert result['step_a'] == 5.
+    assert result['a'] == 5.