Core50 benchmark with ICARL strategy
ICarl strategy returns an error  with Core50 benchmark:
`scenario = CORe50(scenario="nc", run=9)`

The error is:
`AttributeError: 'GenericCLScenario' object has no attribute 'n_classes_per_exp'`

Other nc benchmarks belong to "NCScenario"  class and this issue does not appear.
diff --git a/avalanche/benchmarks/classic/core50.py b/avalanche/benchmarks/classic/core50.py
index 70809cd9..7a58df9d 100644
--- a/avalanche/benchmarks/classic/core50.py
+++ b/avalanche/benchmarks/classic/core50.py
@@ -156,6 +156,20 @@ def CORe50(
         eval_transform=eval_transform,
     )
 
+    if scenario == "nc":
+        n_classes_per_exp = []
+        classes_order = []
+        for exp in benchmark_obj.train_stream:
+            exp_dataset = exp.dataset
+            unique_targets = list(
+                sorted(set(int(x) for x in exp_dataset.targets))  # type: ignore
+            )
+            n_classes_per_exp.append(len(unique_targets))
+            classes_order.extend(unique_targets)
+        setattr(benchmark_obj, "n_classes_per_exp", n_classes_per_exp)
+        setattr(benchmark_obj, "classes_order", classes_order)
+    setattr(benchmark_obj, "n_classes", 50 if object_lvl else 10)
+
     return benchmark_obj
 
 
diff --git a/avalanche/evaluation/metrics/checkpoint.py b/avalanche/evaluation/metrics/checkpoint.py
index b06926cc..f0a0cc44 100644
--- a/avalanche/evaluation/metrics/checkpoint.py
+++ b/avalanche/evaluation/metrics/checkpoint.py
@@ -10,9 +10,11 @@
 ################################################################################
 
 import copy
-from typing import TYPE_CHECKING
+import io
+from typing import TYPE_CHECKING, Optional
 
 from torch import Tensor
+import torch
 
 from avalanche.evaluation import PluginMetric
 from avalanche.evaluation.metric_results import MetricValue, MetricResult
@@ -46,9 +48,9 @@ class WeightCheckpoint(PluginMetric[Tensor]):
         retrieved using the `result` method.
         """
         super().__init__()
-        self.weights = None
+        self.weights: Optional[bytes] = None
 
-    def update(self, weights) -> Tensor:
+    def update(self, weights: bytes):
         """
         Update the weight checkpoint at the current experience.
 
@@ -57,7 +59,7 @@ class WeightCheckpoint(PluginMetric[Tensor]):
         """
         self.weights = weights
 
-    def result(self) -> Tensor:
+    def result(self) -> Optional[bytes]:
         """
         Retrieves the weight checkpoint at the current experience.
 
@@ -75,6 +77,9 @@ class WeightCheckpoint(PluginMetric[Tensor]):
 
     def _package_result(self, strategy) -> "MetricResult":
         weights = self.result()
+        if weights is None:
+            return None
+
         metric_name = get_metric_name(
             self, strategy, add_experience=True, add_task=False
         )
@@ -83,9 +88,13 @@ class WeightCheckpoint(PluginMetric[Tensor]):
         ]
 
     def after_training_exp(self, strategy: "SupervisedTemplate") -> "MetricResult":
-        model_params = copy.deepcopy(strategy.model.parameters())
-        self.update(model_params)
-        return None
+        buff = io.BytesIO()
+        model_params = copy.deepcopy(strategy.model).to("cpu")
+        torch.save(model_params, buff)
+        buff.seek(0)
+        self.update(buff.read())
+
+        return self._package_result(strategy)
 
     def __str__(self):
         return "WeightCheckpoint"
diff --git a/avalanche/logging/text_logging.py b/avalanche/logging/text_logging.py
index 670c20f0..7222410b 100644
--- a/avalanche/logging/text_logging.py
+++ b/avalanche/logging/text_logging.py
@@ -9,7 +9,6 @@
 # Website: avalanche.continualai.org                                           #
 ################################################################################
 import datetime
-import os.path
 import sys
 import warnings
 from typing import List, TYPE_CHECKING, Tuple, Type, Optional, TextIO
@@ -24,7 +23,10 @@ from avalanche.evaluation.metric_utils import stream_type, phase_and_task
 if TYPE_CHECKING:
     from avalanche.training.templates import SupervisedTemplate
 
-UNSUPPORTED_TYPES: Tuple[Type] = (TensorImage,)
+UNSUPPORTED_TYPES: Tuple[Type, ...] = (
+    TensorImage,
+    bytes,
+)
 
 
 class TextLogger(BaseLogger, SupervisedPlugin):
diff --git a/avalanche/logging/wandb_logger.py b/avalanche/logging/wandb_logger.py
index 00120dbd..1bcbd1b0 100644
--- a/avalanche/logging/wandb_logger.py
+++ b/avalanche/logging/wandb_logger.py
@@ -4,21 +4,21 @@
 # See the accompanying LICENSE file for terms.                                 #
 #                                                                              #
 # Date: 25-11-2020                                                             #
-# Author(s): Diganta Misra, Andrea Cossu                                       #
+# Author(s): Diganta Misra, Andrea Cossu, Lorenzo Pellegrini                   #
 # E-mail: contact@continualai.org                                              #
 # Website: www.continualai.org                                                 #
 ################################################################################
 """ This module handles all the functionalities related to the logging of
 Avalanche experiments using Weights & Biases. """
 
-from typing import Union, List, TYPE_CHECKING
+import re
+from typing import Optional, Union, List, TYPE_CHECKING
 from pathlib import Path
 import os
-import errno
+import warnings
 
 import numpy as np
 from numpy import array
-import torch
 from torch import Tensor
 
 from PIL.Image import Image
@@ -37,6 +37,12 @@ if TYPE_CHECKING:
     from avalanche.training.templates import SupervisedTemplate
 
 
+CHECKPOINT_METRIC_NAME = re.compile(
+    r"^WeightCheckpoint\\/(?P<phase_name>\\S+)_phase\\/(?P<stream_name>\\S+)_"
+    r"stream(\\/Task(?P<task_id>\\d+))?\\/Exp(?P<experience_id>\\d+)$"
+)
+
+
 class WandBLogger(BaseLogger, SupervisedPlugin):
     """Weights and Biases logger.
 
@@ -60,18 +66,21 @@ class WandBLogger(BaseLogger, SupervisedPlugin):
         run_name: str = "Test",
         log_artifacts: bool = False,
         path: Union[str, Path] = "Checkpoints",
-        uri: str = None,
+        uri: Optional[str] = None,
         sync_tfboard: bool = False,
         save_code: bool = True,
-        config: object = None,
-        dir: Union[str, Path] = None,
-        params: dict = None,
+        config: Optional[object] = None,
+        dir: Optional[Union[str, Path]] = None,
+        params: Optional[dict] = None,
     ):
         """Creates an instance of the `WandBLogger`.
 
         :param project_name: Name of the W&B project.
         :param run_name: Name of the W&B run.
         :param log_artifacts: Option to log model weights as W&B Artifacts.
+            Note that, in order for model weights to be logged, the
+            :class:`WeightCheckpoint` metric must be added to the
+            evaluation plugin.
         :param path: Path to locally save the model checkpoints.
         :param uri: URI identifier for external storage buckets (GCS, S3).
         :param sync_tfboard: Syncs TensorBoard to the W&B dashboard UI.
@@ -102,6 +111,8 @@ class WandBLogger(BaseLogger, SupervisedPlugin):
     def import_wandb(self):
         try:
             import wandb
+
+            assert hasattr(wandb, "__version__")
         except ImportError:
             raise ImportError('Please run "pip install wandb" to install wandb')
         self.wandb = wandb
@@ -140,7 +151,7 @@ class WandBLogger(BaseLogger, SupervisedPlugin):
         self,
         strategy: "SupervisedTemplate",
         metric_values: List["MetricValue"],
-        **kwargs
+        **kwargs,
     ):
         for val in metric_values:
             self.log_metrics([val])
@@ -151,6 +162,11 @@ class WandBLogger(BaseLogger, SupervisedPlugin):
     def log_single_metric(self, name, value, x_plot):
         self.step = x_plot
 
+        if name.startswith("WeightCheckpoint"):
+            if self.log_artifacts:
+                self._log_checkpoint(name, value, x_plot)
+            return
+
         if isinstance(value, AlternativeValues):
             value = value.best_supported_value(
                 Image,
@@ -192,26 +208,46 @@ class WandBLogger(BaseLogger, SupervisedPlugin):
         elif isinstance(value, TensorImage):
             self.wandb.log({name: self.wandb.Image(array(value))}, step=self.step)
 
-        elif name.startswith("WeightCheckpoint"):
-            if self.log_artifacts:
-                cwd = os.getcwd()
-                ckpt = os.path.join(cwd, self.path)
-                try:
-                    os.makedirs(ckpt)
-                except OSError as e:
-                    if e.errno != errno.EEXIST:
-                        raise
-                suffix = ".pth"
-                dir_name = os.path.join(ckpt, name + suffix)
-                artifact_name = os.path.join("Models", name + suffix)
-                if isinstance(value, Tensor):
-                    torch.save(value, dir_name)
-                    name = os.path.splittext(self.checkpoint)
-                    artifact = self.wandb.Artifact(name, type="model")
-                    artifact.add_file(dir_name, name=artifact_name)
-                    self.wandb.run.log_artifact(artifact)
-                    if self.uri is not None:
-                        artifact.add_reference(self.uri, name=artifact_name)
+    def _log_checkpoint(self, name, value, x_plot):
+        assert self.wandb is not None
+
+        # Example: 'WeightCheckpoint/train_phase/train_stream/Task000/Exp000'
+        name_match = CHECKPOINT_METRIC_NAME.match(name)
+        if name_match is None:
+            warnings.warn(f"Checkpoint metric has unsupported name {name}.")
+            return
+        # phase_name: str = name_match['phase_name']
+        # stream_name: str = name_match['stream_name']
+        task_id: Optional[int] = (
+            int(name_match["task_id"]) if name_match["task_id"] is not None else None
+        )
+        experience_id: int = int(name_match["experience_id"])
+        assert experience_id >= 0
+
+        cwd = Path.cwd()
+        checkpoint_directory = cwd / self.path
+        checkpoint_directory.mkdir(parents=True, exist_ok=True)
+
+        checkpoint_name = "Model_{}".format(experience_id)
+        checkpoint_file_name = checkpoint_name + ".pth"
+        checkpoint_path = checkpoint_directory / checkpoint_file_name
+        artifact_name = "Models/" + checkpoint_file_name
+
+        # Write the checkpoint blob
+        with open(checkpoint_path, "wb") as f:
+            f.write(value)
+
+        metadata = {
+            "experience": experience_id,
+            "x_step": x_plot,
+            **({"task_id": task_id} if task_id is not None else {}),
+        }
+
+        artifact = self.wandb.Artifact(checkpoint_name, type="model", metadata=metadata)
+        artifact.add_file(str(checkpoint_path), name=artifact_name)
+        self.wandb.run.log_artifact(artifact)
+        if self.uri is not None:
+            artifact.add_reference(self.uri, name=artifact_name)
 
     def __getstate__(self):
         state = self.__dict__.copy()
diff --git a/avalanche/training/plugins/ewc.py b/avalanche/training/plugins/ewc.py
index f3c2f05a..6c829cf1 100644
--- a/avalanche/training/plugins/ewc.py
+++ b/avalanche/training/plugins/ewc.py
@@ -121,6 +121,7 @@ class EWCPlugin(SupervisedPlugin):
             strategy.experience.dataset,
             strategy.device,
             strategy.train_mb_size,
+            num_workers=kwargs.get("num_workers", 0),
         )
         self.update_importances(importances, exp_counter)
         self.saved_params[exp_counter] = copy_params_dict(strategy.model)
@@ -129,7 +130,7 @@ class EWCPlugin(SupervisedPlugin):
             del self.saved_params[exp_counter - 1]
 
     def compute_importances(
-        self, model, criterion, optimizer, dataset, device, batch_size
+        self, model, criterion, optimizer, dataset, device, batch_size, num_workers=0
     ) -> Dict[str, ParamData]:
         """
         Compute EWC importance matrix for each parameter
@@ -153,7 +154,12 @@ class EWCPlugin(SupervisedPlugin):
         # list of list
         importances = zerolike_params_dict(model)
         collate_fn = dataset.collate_fn if hasattr(dataset, "collate_fn") else None
-        dataloader = DataLoader(dataset, batch_size=batch_size, collate_fn=collate_fn)
+        dataloader = DataLoader(
+            dataset,
+            batch_size=batch_size,
+            collate_fn=collate_fn,
+            num_workers=num_workers,
+        )
         for i, batch in enumerate(dataloader):
             # get only input, target and task_id from the batch
             x, y, task_labels = batch[0], batch[1], batch[-1]
diff --git a/examples/multihead.py b/examples/multihead.py
index c4facb55..c0177418 100644
--- a/examples/multihead.py
+++ b/examples/multihead.py
@@ -69,8 +69,8 @@ def main(args):
 
     # train and test loop
     for train_task in train_stream:
-        strategy.train(train_task)
-        strategy.eval(test_stream)
+        strategy.train(train_task, num_workers=4)
+        strategy.eval(test_stream, num_workers=4)
 
 
 if __name__ == "__main__":
diff --git a/examples/wandb_logger.py b/examples/wandb_logger.py
index 5ba9b94d..5404ea6b 100644
--- a/examples/wandb_logger.py
+++ b/examples/wandb_logger.py
@@ -24,6 +24,7 @@ from torchvision.transforms import ToTensor, RandomCrop
 
 from avalanche.benchmarks import nc_benchmark
 from avalanche.benchmarks.datasets.dataset_utils import default_dataset_location
+from avalanche.evaluation.metrics.checkpoint import WeightCheckpoint
 from avalanche.logging import InteractiveLogger, WandBLogger
 from avalanche.training.plugins import EvaluationPlugin
 from avalanche.evaluation.metrics import (
@@ -83,7 +84,11 @@ def main(args):
 
     interactive_logger = InteractiveLogger()
     wandb_logger = WandBLogger(
-        project_name=args.project, run_name=args.run, config=vars(args)
+        project_name=args.project,
+        run_name=args.run,
+        log_artifacts=args.artifacts,
+        path=args.path if args.path else None,
+        config=vars(args),
     )
 
     eval_plugin = EvaluationPlugin(
@@ -120,6 +125,7 @@ def main(args):
         ),
         disk_usage_metrics(minibatch=True, epoch=True, experience=True, stream=True),
         MAC_metrics(minibatch=True, epoch=True, experience=True),
+        WeightCheckpoint(),
         loggers=[interactive_logger, wandb_logger],
     )
 
@@ -157,9 +163,22 @@ if __name__ == "__main__":
         default=0,
         help="Select zero-indexed cuda device. -1 to use CPU.",
     )
-    parser.add_argument("--run", type=str, help="Provide a run name for WandB")
     parser.add_argument(
         "--project", type=str, help="Define the name of the WandB project"
     )
+    parser.add_argument("--run", type=str, help="Provide a run name for WandB")
+    parser.add_argument(
+        "--artifacts",
+        default=False,
+        action="store_true",
+        help="Log Model Checkpoints as W&B Artifacts",
+    )
+    parser.add_argument(
+        "--path",
+        type=str,
+        default="Checkpoint",
+        help="Local path to save the model checkpoints",
+    )
+
     args = parser.parse_args()
     main(args)
diff --git a/tests/test_core50.py b/tests/test_core50.py
index fe4e4c73..13f91a2c 100644
--- a/tests/test_core50.py
+++ b/tests/test_core50.py
@@ -38,6 +38,11 @@ class CORe50Test(unittest.TestCase):
         classes_in_test = benchmark_instance.classes_in_experience["test"][0]
         self.assertSetEqual(set(range(50)), set(classes_in_test))
 
+        # Regression tests for issue #774
+        self.assertSequenceEqual([10] + ([5] * 8), benchmark_instance.n_classes_per_exp)
+        self.assertSetEqual(set(range(50)), set(benchmark_instance.classes_order))
+        self.assertEqual(50, len(benchmark_instance.classes_order))
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/tests/test_models.py b/tests/test_models.py
index 5103d39b..1d7f8af5 100644
--- a/tests/test_models.py
+++ b/tests/test_models.py
@@ -1,6 +1,7 @@
 import sys
 import os
 import copy
+import tempfile
 
 import unittest
 
@@ -650,10 +651,13 @@ class NCMClassifierTest(unittest.TestCase):
                 ),
             }
         )
-        torch.save(classifier.state_dict(), "ncm.pt")
-        del classifier
-        classifier = NCMClassifier()
-        check = torch.load("ncm.pt")
+
+        with tempfile.TemporaryFile() as tmpfile:
+            torch.save(classifier.state_dict(), tmpfile)
+            del classifier
+            classifier = NCMClassifier()
+            tmpfile.seek(0)
+            check = torch.load(tmpfile)
         classifier.load_state_dict(check)
         assert classifier.class_means.shape == (3, 5)
         assert (classifier.class_means[0] == 0).all()
['examples/wandb_logger.py', 'tests/test_core50.py', 'avalanche/training/plugins/ewc.py', 'avalanche/evaluation/metrics/checkpoint.py', 'examples/multihead.py', 'tests/test_models.py', 'avalanche/logging/wandb_logger.py', 'avalanche/benchmarks/classic/core50.py', 'avalanche/logging/text_logging.py']