chore(docs): add usage in supervised_training_steps (#1661)

* chore(docs): add examples in supervised_training_steps

* fix: add import from

ignite/engine/__init__.py

@@ -69,6 +69,17 @@ def supervised_training_step(
     Returns:
         Callable: update function.
 
+    Example::
+
+        from ignite.engine import Engine, supervised_training_step
+
+        model = ...
+        optimizer = ...
+        loss_fn = ...
+
+        update_fn = supervised_training_step(model, optimizer, loss_fn, 'cuda')
+        trainer = Engine(update_fn)
+
     .. versionadded:: 0.5.0
     """
 
@@ -115,6 +126,18 @@ def supervised_training_step_amp(
     Returns:
         Callable: update function
 
+    Example::
+
+        from ignite.engine import Engine, supervised_training_step_amp
+
+        model = ...
+        optimizer = ...
+        loss_fn = ...
+        scaler = torch.cuda.amp.GradScaler(2**10)
+
+        update_fn = supervised_training_step_amp(model, optimizer, loss_fn, 'cuda', scaler=scaler)
+        trainer = Engine(update_fn)
+
     .. versionadded:: 0.5.0
     """
 
@@ -170,6 +193,17 @@ def supervised_training_step_apex(
     Returns:
         Callable: update function.
 
+    Example::
+
+        from ignite.engine import Engine, supervised_training_step_apex
+
+        model = ...
+        optimizer = ...
+        loss_fn = ...
+
+        update_fn = supervised_training_step_apex(model, optimizer, loss_fn, 'cuda')
+        trainer = Engine(update_fn)
+
     .. versionadded:: 0.5.0
     """
 
@@ -220,6 +254,17 @@ def supervised_training_step_tpu(
     Returns:
         Callable: update function.
 
+    Example::
+
+        from ignite.engine import Engine, supervised_training_step_tpu
+
+        model = ...
+        optimizer = ...
+        loss_fn = ...
+
+        update_fn = supervised_training_step_tpu(model, optimizer, loss_fn, 'xla')
+        trainer = Engine(update_fn)
+
     .. versionadded:: 0.5.0
     """
     try: