Exposed EventEnum in docs and added import statement in its examples (#1345) (#1353)

Author: harsh8398

README.md

@@ -210,6 +210,9 @@ Examples
 
 Custom events related to backward and optimizer step calls:
 ```python
+from ignite.engine import EventEnum
+
+
 class BackpropEvents(EventEnum):
     BACKWARD_STARTED = 'backward_started'
     BACKWARD_COMPLETED = 'backward_completed'

docs/source/concepts.rst

@@ -234,6 +234,8 @@ and be registered with :meth:`~ignite.engine.engine.Engine.register_events` in a
 
 .. code-block:: python
 
+    from ignite.engine import EventEnum
+
     class CustomEvents(EventEnum):
         """
         Custom events defined by user

docs/source/engine.rst

@@ -92,6 +92,8 @@ ignite.engine.events
 .. autoclass:: Events
    :members:
 
+.. autoclass:: EventEnum
+
 .. autoclass:: State
 
 .. autoclass:: RemovableEventHandle

docs/source/faq.rst

@@ -47,6 +47,8 @@ flexibility to the user to allow for this:
 
 .. code-block:: python
 
+    from ignite.engine import EventEnum
+    
     class BackpropEvents(EventEnum):
         """
         Events based on back propagation

ignite/engine/events.py

@@ -142,6 +142,36 @@ def __init__(self, *args, **kwargs):
 
 
 class EventEnum(CallableEventWithFilter, Enum):
+    """Base class for all :class:`~ignite.engine.events.Events`. User defined custom events should also inherit
+    this class. For example, Custom events based on the loss calculation and backward pass can be created as follows:
+
+        .. code-block:: python
+
+            from ignite.engine import EventEnum
+
+            class BackpropEvents(EventEnum):
+                BACKWARD_STARTED = 'backward_started'
+                BACKWARD_COMPLETED = 'backward_completed'
+                OPTIM_STEP_COMPLETED = 'optim_step_completed'
+
+            def update(engine, batch):
+                # ...
+                loss = criterion(y_pred, y)
+                engine.fire_event(BackpropEvents.BACKWARD_STARTED)
+                loss.backward()
+                engine.fire_event(BackpropEvents.BACKWARD_COMPLETED)
+                optimizer.step()
+                engine.fire_event(BackpropEvents.OPTIM_STEP_COMPLETED)
+                # ...
+
+            trainer = Engine(update)
+            trainer.register_events(*BackpropEvents)
+
+            @trainer.on(BackpropEvents.BACKWARD_STARTED)
+            def function_before_backprop(engine):
+                # ...
+    """
+
     pass