Repository: Project-MONAI/MONAI

Issue:
sphinx documentation warnings
**Describe the bug**
building the documentation locally generates a couple of warnings:
```bash
building [mo]: targets for 0 po files that are out of date
building [html]: targets for 16 source files that are out of date
updating environment: [new config] 16 added, 0 changed, 0 removed
/usr/local/anaconda3/envs/py36/lib/python3.6/site-packages/recommonmark/parser.py:75: UserWarning: Container node skipped: type=document
  warn("Container node skipped: type={0}".format(mdnode.t))
/usr/local/anaconda3/envs/py36/lib/python3.6/site-packages/recommonmark/parser.py:75: UserWarning: Container node skipped: type=document
  warn("Container node skipped: type={0}".format(mdnode.t))
reading sources... [100%] visualize                                                                                            
WARNING: autodoc: failed to import class 'handlers.CheckpointLoader' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.CheckpointSaver' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.ClassificationSaver' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.MeanDice' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.ROCAUC' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.MetricLogger' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.SegmentationSaver' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.StatsHandler' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.TensorBoardStatsHandler' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.LrScheduleHandler' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
WARNING: autodoc: failed to import class 'handlers.ValidationHandler' from module 'monai'; the following exception was raised:
cannot import name 'RemovableEventHandle'
looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents... done
writing output... [100%] visualize                              
```

**To Reproduce**
Steps to reproduce the behavior:
1. Go to `MONAI/docs`
2. ` pip install -U -r requirements.txt` to install doc deps
3. Run `make clean; make html`

**Expected behavior**
no warnings in the building log

**Environment (please complete the following information):**
```
MONAI version: 0.2.0+77.g783f6a1
Python version: 3.6.10 |Anaconda, Inc.| (default, Mar 25 2020, 18:53:43)  [GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)]
Numpy version: 1.19.1
Pytorch version: 1.6.0

Optional dependencies:
Pytorch Ignite version: 0.3.0
Nibabel version: 3.1.1
scikit-image version: 0.17.2
Pillow version: 7.2.0
Tensorboard version: 2.3.0
```



Discussion:

Resolving patch:
diff --git a/monai/handlers/checkpoint_loader.py b/monai/handlers/checkpoint_loader.py
--- a/monai/handlers/checkpoint_loader.py
+++ b/monai/handlers/checkpoint_loader.py
@@ -19,10 +19,9 @@
 Events, _ = optional_import("ignite.engine", "0.3.0", exact_version, "Events")
 Checkpoint, _ = optional_import("ignite.handlers", "0.3.0", exact_version, "Checkpoint")
 if TYPE_CHECKING:
-    from ignite.engine import Engine, RemovableEventHandle
+    from ignite.engine import Engine
 else:
     Engine, _ = optional_import("ignite.engine", "0.3.0", exact_version, "Engine")
-    RemovableEventHandle, _ = optional_import("ignite.engine", "0.3.0", exact_version, "RemovableEventHandle")
 
 
 class CheckpointLoader:
@@ -54,14 +53,14 @@ def __init__(self, load_path: str, load_dict: Dict, name: Optional[str] = None)
 
         self._name = name
 
-    def attach(self, engine: Engine) -> RemovableEventHandle:
+    def attach(self, engine: Engine) -> None:
         """
         Args:
             engine: Ignite Engine, it can be a trainer, validator or evaluator.
         """
         if self._name is None:
             self.logger = engine.logger
-        return engine.add_event_handler(Events.STARTED, self)
+        engine.add_event_handler(Events.STARTED, self)
 
     def __call__(self, engine: Engine) -> None:
         """
diff --git a/monai/handlers/metric_logger.py b/monai/handlers/metric_logger.py
--- a/monai/handlers/metric_logger.py
+++ b/monai/handlers/metric_logger.py
@@ -16,10 +16,9 @@
 
 Events, _ = optional_import("ignite.engine", "0.3.0", exact_version, "Events")
 if TYPE_CHECKING:
-    from ignite.engine import Engine, RemovableEventHandle
+    from ignite.engine import Engine
 else:
     Engine, _ = optional_import("ignite.engine", "0.3.0", exact_version, "Engine")
-    RemovableEventHandle, _ = optional_import("ignite.engine", "0.3.0", exact_version, "RemovableEventHandle")
 
 
 class MetricLogger:
@@ -29,12 +28,12 @@ def __init__(self, loss_transform: Callable = lambda x: x, metric_transform: Cal
         self.loss: List = []
         self.metrics: DefaultDict = defaultdict(list)
 
-    def attach(self, engine: Engine) -> RemovableEventHandle:
+    def attach(self, engine: Engine) -> None:
         """
         Args:
             engine: Ignite Engine, it can be a trainer, validator or evaluator.
         """
-        return engine.add_event_handler(Events.ITERATION_COMPLETED, self)
+        engine.add_event_handler(Events.ITERATION_COMPLETED, self)
 
     def __call__(self, engine: Engine) -> None:
         """


Test patch: