Fixing tutorial with DataLoader

Updating the tutorial and README with more relevant/correct information. Minor fix to one part of `MapDataPipe` documentation as well.

Fixes #352

README.md

@@ -177,17 +177,30 @@ Q: What should I do if the existing set of DataPipes does not do what I need?
 A: You can
 [implement your own custom DataPipe](https://pytorch.org/data/main/tutorial.html#implementing-a-custom-datapipe). If you
 believe your use case is common enough such that the community can benefit from having your custom DataPipe added to
-this library, feel free to open a GitHub issue.
+this library, feel free to open a GitHub issue. We will be happy to discuss!
 
-Q: What happens when the `Shuffler`/`Batcher` DataPipes are used with DataLoader?
+Q: What happens when the `Shuffler` DataPipe is used with DataLoader?
 
-A: If you choose those DataPipes while setting `shuffle=True`/`batch_size>1` for DataLoader, your samples will be
-shuffled/batched more than once. You should choose one or the other.
+A. In order to enable shuffling, you need to add a `Shuffler` to your DataPipe line. Then, by default, shuffling will
+happen at the point where you specified as long as you do not set `shuffle=False` within DataLoader.
+
+Q: What happens when the `Batcher` DataPipe is used with DataLoader?
+
+A: If you choose to use `Batcher` while setting `batch_size > 1` for DataLoader, your samples will be batched more than
+once. You should choose one or the other.
+
+Q: Why are there fewer built-in `MapDataPipes` than `IterDataPipes`?
+
+A: By design, there are fewer `MapDataPipes` than `IterDataPipes` to avoid duplicate implementations of the same
+functionalities as `MapDataPipe`. We encourage users to use the built-in `IterDataPipe` for various functionalities, and
+convert it to `MapDataPipe` as needed.
 
 Q: How is multiprocessing handled with DataPipes?
 
 A: Multi-process data loading is still handled by DataLoader, see the
 [DataLoader documentation for more details](https://pytorch.org/docs/stable/data.html#single-and-multi-process-data-loading).
+If you would like to shard data across processes, use `ShardingFilter` and provide a `worker_init_fn` as shown in the
+[tutorial](https://pytorch.org/data/beta/tutorial.html#working-with-dataloader).
 
 Q: What is the upcoming plan for DataLoader?
 

docs/source/torchdata.datapipes.map.rst

@@ -15,9 +15,9 @@ corresponding label from a folder on the disk.
 
 By design, there are fewer ``MapDataPipe`` than ``IterDataPipe`` to avoid duplicate implementations of the same
 functionalities as ``MapDataPipe``. We encourage users to use the built-in ``IterDataPipe`` for various functionalities,
-and convert it to ``MapDataPipe`` as needed using ``MapToIterConverter`` or ``.to_iter_datapipe()``.
-If you have any question about usage or best practices while using `MapDataPipe`, feel free to ask on the PyTorch forum
-under the `'data' category <https://discuss.pytorch.org/c/data/37>`_.
+and convert it to ``MapDataPipe`` as needed using :class:`.IterToMapConverter` or ``.to_map_datapipe()``.
+If you have any question about usage or best practices while using ``MapDataPipe``, feel free to ask on the PyTorch
+forum under the `'data' category <https://discuss.pytorch.org/c/data/37>`_.
 
 We are open to add additional ``MapDataPipe`` where the operations can be lazily executed and ``__len__`` can be
 known in advance. Feel free to make suggestions with description of your use case in

docs/source/tutorial.rst

@@ -80,23 +80,33 @@ For this example, we will first have a helper function that generates some CSV f
             row_data['label'] = random.randint(0, 9)
             writer.writerow(row_data)
 
-Next, we will build our DataPipes to read and parse through the generated CSV files:
+Next, we will build our DataPipes to read and parse through the generated CSV files. Note that we prefer to have
+pass defined functions to DataPipes rather than lambda functions because the formers are serializable with `pickle`.
 
 .. code:: python
 
     import numpy as np
     import torchdata.datapipes as dp
 
+    def filter_for_data(filename):
+        return "sample_data" in filename and filename.endswith(".csv")
+
+    def row_processer(row):
+        return {"label": np.array(row[0], np.int32), "data": np.array(row[1:], dtype=np.float64)}
+
     def build_datapipes(root_dir="."):
         datapipe = dp.iter.FileLister(root_dir)
-        datapipe = datapipe.filter(filter_fn=lambda filename: "sample_data" in filename and filename.endswith(".csv"))
-        datapipe = dp.iter.FileOpener(datapipe, mode='rt')
+        datapipe = datapipe.filter(filter_fn=filter_for_data)
+        datapipe = datapipe.open_files(mode='rt')
         datapipe = datapipe.parse_csv(delimiter=",", skip_lines=1)
-        datapipe = datapipe.map(lambda row: {"label": np.array(row[0], np.int32),
-                                             "data": np.array(row[1:], dtype=np.float64)})
+        # Shuffle will happen as long as you do NOT set `shuffle=False` later in the DataLoader
+        datapipe = datapipe.shuffle()
+        datapipe = datapipe.map(row_processer)
         return datapipe
 
-Lastly, we will put everything together in ``'__main__'`` and pass the DataPipe into the DataLoader.
+Lastly, we will put everything together in ``'__main__'`` and pass the DataPipe into the DataLoader. Note that
+if you choose to use `Batcher` while setting `batch_size > 1` for DataLoader, your samples will be
+batched more than once. You should choose one or the other.
 
 .. code:: python
 
@@ -105,20 +115,67 @@ Lastly, we will put everything together in ``'__main__'`` and pass the DataPipe
     if __name__ == '__main__':
         num_files_to_generate = 3
         for i in range(num_files_to_generate):
-            generate_csv(file_label=i)
+            generate_csv(file_label=i, num_rows=10, num_features=3)
         datapipe = build_datapipes()
-        dl = DataLoader(dataset=datapipe, batch_size=50, shuffle=True)
+        dl = DataLoader(dataset=datapipe, batch_size=5, num_workers=2)
         first = next(iter(dl))
         labels, features = first['label'], first['data']
         print(f"Labels batch shape: {labels.size()}")
         print(f"Feature batch shape: {features.size()}")
+        print(f"{labels = }\n{features = }")
+        n_sample = 0
+        for row in iter(dl):
+            n_sample += 1
+        print(f"{n_sample = }")
 
 The following statements will be printed to show the shapes of a single batch of labels and features.
 
 .. code::
 
-    Labels batch shape: 50
-    Feature batch shape: torch.Size([50, 20])
+    Labels batch shape: torch.Size([5])
+    Feature batch shape: torch.Size([5, 3])
+    labels = tensor([8, 9, 5, 9, 7], dtype=torch.int32)
+    features = tensor([[0.2867, 0.5973, 0.0730],
+            [0.7890, 0.9279, 0.7392],
+            [0.8930, 0.7434, 0.0780],
+            [0.8225, 0.4047, 0.0800],
+            [0.1655, 0.0323, 0.5561]], dtype=torch.float64)
+    n_sample = 12
+
+The reason why ``n_sample = 12`` is because ``ShardingFilter`` (``datapipe.sharding_filter()``) was not used, such that
+each worker will independently return all samples. In this case, there are 10 rows per file and 3 files, with a
+batch size of 5, that gives us 6 batches per worker. With 2 workers, we get 12 total batches from the ``DataLoader``.
+
+In order for DataPipe sharding to work with ``DataLoader``, we need to add the following. It is crucial to add
+`ShardingFilter` after `Shuffler` to ensure that all worker processes have the same order of data for sharding.
+
+.. code:: python
+
+    def build_datapipes(root_dir="."):
+        datapipe = ...
+        # Add the following line to `build_datapipes`
+        # Note that it is somewhere after `Shuffler` in the DataPipe line
+        datapipe = datapipe.sharding_filter()
+        return datapipe
+
+    def worker_init_fn(worker_id):
+        info = torch.utils.data.get_worker_info()
+        num_workers = info.num_workers
+        datapipe = info.dataset
+        torch.utils.data.graph_settings.apply_sharding(datapipe, num_workers, worker_id)
+
+    # Pass `worker_init_fn` into `DataLoader` within '__main__'
+    ...
+    dl = DataLoader(dataset=datapipe, shuffle=True, batch_size=5, num_workers=2, worker_init_fn=worker_init_fn)
+    ...
+
+When we re-run, we will get:
+
+.. code::
+
+    ...
+    n_sample = 6
+
 
 You can find more DataPipe implementation examples for various research domains `on this page <torchexamples.html>`_.