docs(imperative): add example of dataset

GitOrigin-RevId: 70f2a513cfb5c6e48b30e0e56bf469f3ac99a86a

imperative/python/megengine/data/dataloader.py

@@ -68,6 +68,10 @@ class DataLoader:
             batch from workers. Default: 0
         preload: whether to enable the preloading strategy of the dataloader. 
             When enabling, the dataloader will preload one batch to the device memory to speed up the whole training process.
+        parallel_stream: whether to splitting workload across all workers when dataset is streamdataset and num_workers > 0.
+            When enabling, each worker will collect data from different dataset in order to speed up the whole loading process.
+            See ref:`streamdataset-example` for more details
+
 
     .. admonition:: The effect of enabling preload
        :class: warning

imperative/python/megengine/data/dataset/meta_dataset.py

@@ -58,6 +58,33 @@ class StreamDataset(Dataset):
     r"""An abstract class for stream data.
 
     __iter__ method is aditionally needed.
+
+    Examples:
+
+        .. code-block:: python
+
+            from megengine.data.dataset import StreamDataset
+            from megengine.data.dataloader import DataLoader, get_worker_info
+            from megengine.data.sampler import StreamSampler
+
+            class MyStream(StreamDataset):
+                def __init__(self):
+                    self.data = [iter([1, 2, 3]), iter([4, 5, 6]), iter([7, 8, 9])]
+                def __iter__(self):
+                    worker_info = get_worker_info()
+                    data_iter = self.data[worker_info.idx]
+                    while True:
+                        yield next(data_iter)
+
+            dataloader = DataLoader(
+                dataset = MyStream(),
+                sampler = StreamSampler(batch_size=2),
+                num_workers=3,
+                parallel_stream = True,
+            )
+
+            for step, data in enumerate(dataloader):
+                print(data)
     """
 
     @abstractmethod
@@ -80,6 +107,29 @@ class ArrayDataset(Dataset):
 
     One or more numpy arrays are needed to initiate the dataset.
     And the dimensions represented sample number are expected to be the same.
+
+    Examples:
+
+        .. code-block:: python
+
+            from megengine.data.dataset import ArrayDataset
+            from megengine.data.dataloader import DataLoader
+            from megengine.data.sampler import SequentialSampler
+
+            rand_data = np.random.randint(0, 255, size=(sample_num, 1, 32, 32), dtype=np.uint8)
+            label = np.random.randint(0, 10, size=(sample_num,), dtype=int)
+            dataset = ArrayDataset(rand_data, label)
+            seque_sampler = SequentialSampler(dataset, batch_size=2)
+
+            dataloader = DataLoader(
+                dataset,
+                sampler = seque_sampler,
+                num_workers=3,
+            )
+
+            for step, data in enumerate(dataloader):
+                print(data)
+
     """
 
     def __init__(self, *arrays):