[skip ci] Add doctest for LR Schedulers (#2384)

* [skip ci] doctest for CosineAnnealingScheduler

* doctest for ConcatScheduler

Co-authored-by: Desroziers <sylvain.desroziers@michelin.com>

ignite/handlers/param_scheduler.py

@@ -365,8 +365,6 @@ class LinearCyclicalScheduler(CyclicalScheduler):
 
         .. testcode:: 1
 
-            from ignite.handlers.param_scheduler import LinearCyclicalScheduler
-
             # Linearly increases the learning rate from 0.0 to 1.0 and back to 0.0
             # over a cycle of 4 iterations
             scheduler = LinearCyclicalScheduler(default_optimizer, "lr", 0.0, 1.0, 4)
@@ -389,8 +387,6 @@ def print_lr():
 
         .. testcode:: 2
 
-            from ignite.handlers.param_scheduler import LinearCyclicalScheduler
-
             optimizer = torch.optim.SGD(
                 [
                     {"params": default_model.base.parameters(), "lr": 0.001},
@@ -400,19 +396,19 @@ def print_lr():
 
             # Linearly increases the learning rate from 0.0 to 1.0 and back to 0.0
             # over a cycle of 4 iterations
-            scheduler1 = LinearCyclicalScheduler(optimizer, "lr", 0.0, 1.0, 4, param_group_index=0)
+            scheduler1 = LinearCyclicalScheduler(optimizer, "lr (base)", 0.0, 1.0, 4, param_group_index=0)
 
-            # Linearly increases the learning rate from 1.0 to 0.0 and back to 0.1
+            # Linearly increases the learning rate from 0.0 to 0.1 and back to 0.0
             # over a cycle of 4 iterations
-            scheduler2 = LinearCyclicalScheduler(optimizer, "lr", 0.0, 0.1, 4, param_group_index=1)
+            scheduler2 = LinearCyclicalScheduler(optimizer, "lr (fc)", 0.0, 0.1, 4, param_group_index=1)
 
             default_trainer.add_event_handler(Events.ITERATION_STARTED, scheduler1)
             default_trainer.add_event_handler(Events.ITERATION_STARTED, scheduler2)
 
             @default_trainer.on(Events.ITERATION_COMPLETED)
             def print_lr():
-                print(optimizer.param_groups[0]["lr"],
-                      optimizer.param_groups[1]["lr"])
+                print(optimizer.param_groups[0]["lr (base)"],
+                      optimizer.param_groups[1]["lr (fc)"])
 
             default_trainer.run([0] * 9, max_epochs=1)
 
@@ -460,33 +456,67 @@ class CosineAnnealingScheduler(CyclicalScheduler):
         usually be the number of batches in an epoch.
 
     Examples:
-        .. code-block:: python
 
-            from ignite.handlers.param_scheduler import CosineAnnealingScheduler
+        .. testsetup:: *
 
-            scheduler = CosineAnnealingScheduler(optimizer, 'lr', 1e-1, 1e-3, len(train_loader))
-            trainer.add_event_handler(Events.ITERATION_STARTED, scheduler)
-            #
-            # Anneals the learning rate from 1e-1 to 1e-3 over the course of 1 epoch.
-            #
+            default_trainer = get_default_trainer()
 
-        .. code-block:: python
+        .. testcode:: 1
+
+            # CosineAnnealing increases the learning rate from 0.0 to 1.0
+            # over a cycle of 4 iterations
+            scheduler = CosineAnnealingScheduler(default_optimizer, "lr", 0.0, 1.0, 4)
+
+            default_trainer.add_event_handler(Events.ITERATION_STARTED, scheduler)
+
+            @default_trainer.on(Events.ITERATION_COMPLETED)
+            def print_lr():
+                print(default_optimizer.param_groups[0]["lr"])
+
+            default_trainer.run([0] * 9, max_epochs=1)
 
-            from ignite.handlers.param_scheduler import CosineAnnealingScheduler
-            from ignite.handlers.param_scheduler import LinearCyclicalScheduler
+        .. testoutput:: 1
 
-            optimizer = SGD(
+            0.0
+            0.1464...
+            0.4999...
+            0.8535...
+            ...
+
+        .. testcode:: 2
+
+            optimizer = torch.optim.SGD(
                 [
-                    {"params": model.base.parameters(), 'lr': 0.001},
-                    {"params": model.fc.parameters(), 'lr': 0.01},
+                    {"params": default_model.base.parameters(), "lr": 0.001},
+                    {"params": default_model.fc.parameters(), "lr": 0.01},
                 ]
             )
 
-            scheduler1 = LinearCyclicalScheduler(optimizer, 'lr', 1e-7, 1e-5, len(train_loader), param_group_index=0)
-            trainer.add_event_handler(Events.ITERATION_STARTED, scheduler1, "lr (base)")
+            # CosineAnnealing increases the learning rate from 0.0 to 1.0
+            # over a cycle of 4 iterations
+            scheduler1 = CosineAnnealingScheduler(optimizer, "lr (base)", 0.0, 1.0, 4, param_group_index=0)
 
-            scheduler2 = CosineAnnealingScheduler(optimizer, 'lr', 1e-5, 1e-3, len(train_loader), param_group_index=1)
-            trainer.add_event_handler(Events.ITERATION_STARTED, scheduler2, "lr (fc)")
+            # CosineAnnealing increases the learning rate from 0.0 to 0.1
+            # over a cycle of 4 iterations
+            scheduler2 = CosineAnnealingScheduler(optimizer, "lr (fc)", 0.0, 0.1, 4, param_group_index=1)
+
+            default_trainer.add_event_handler(Events.ITERATION_STARTED, scheduler1)
+            default_trainer.add_event_handler(Events.ITERATION_STARTED, scheduler2)
+
+            @default_trainer.on(Events.ITERATION_COMPLETED)
+            def print_lr():
+                print(optimizer.param_groups[0]["lr (base)"],
+                      optimizer.param_groups[1]["lr (fc)"])
+
+            default_trainer.run([0] * 9, max_epochs=1)
+
+        .. testoutput:: 2
+
+            0.0 0.0
+            0.1464... 0.01464...
+            0.4999... 0.04999...
+            0.8535... 0.08535...
+            ...
 
     .. [Smith17] Smith, Leslie N. "Cyclical learning rates for training neural networks."
                  Applications of Computer Vision (WACV), 2017 IEEE Winter Conference on. IEEE, 2017
@@ -513,21 +543,39 @@ class ConcatScheduler(ParamScheduler):
             `engine.state.param_history`, (default=False).
 
     Examples:
-        .. code-block:: python
 
-            from ignite.handlers.param_scheduler import ConcatScheduler
-            from ignite.handlers.param_scheduler import LinearCyclicalScheduler
-            from ignite.handlers.param_scheduler import CosineAnnealingScheduler
+        .. testsetup::
 
-            scheduler_1 = LinearCyclicalScheduler(optimizer, "lr", start_value=0.1, end_value=0.5, cycle_size=60)
-            scheduler_2 = CosineAnnealingScheduler(optimizer, "lr", start_value=0.5, end_value=0.01, cycle_size=60)
+            default_trainer = get_default_trainer()
+
+        .. testcode::
 
-            combined_scheduler = ConcatScheduler(schedulers=[scheduler_1, scheduler_2], durations=[30, ])
-            trainer.add_event_handler(Events.ITERATION_STARTED, combined_scheduler)
-            #
-            # Sets the Learning rate linearly from 0.1 to 0.5 over 30 iterations. Then
-            # starts an annealing schedule from 0.5 to 0.01 over 60 iterations.
+            scheduler_1 = LinearCyclicalScheduler(default_optimizer, "lr", 0.0, 1.0, 8)
+            scheduler_2 = CosineAnnealingScheduler(default_optimizer, "lr", 1.0, 0.2, 4)
+
+            # Sets the Learning rate linearly from 0.0 to 1.0 over 4 iterations. Then
+            # starts an annealing schedule from 1.0 to 0.2 over the next 4 iterations.
             # The annealing cycles are repeated indefinitely.
+            combined_scheduler = ConcatScheduler(schedulers=[scheduler_1, scheduler_2], durations=[4, ])
+
+            default_trainer.add_event_handler(Events.ITERATION_STARTED, combined_scheduler)
+
+            @default_trainer.on(Events.ITERATION_COMPLETED)
+            def print_lr():
+                print(default_optimizer.param_groups[0]["lr"])
+
+            default_trainer.run([0] * 8, max_epochs=1)
+
+        .. testoutput::
+
+            0.0
+            0.25
+            0.5
+            0.75
+            1.0
+            0.8828...
+            0.6000...
+            0.3171...
 
     .. versionadded:: 0.4.5
     """