adding stateful dataloader docs

docs/source/index.rst

@@ -36,6 +36,7 @@ Features described in this documentation are classified by release status:
    :maxdepth: 2
    :caption: API Reference:
 
+   torchdata.stateful_dataloader.rst
    torchdata.datapipes.iter.rst
    torchdata.datapipes.map.rst
    torchdata.datapipes.utils.rst

docs/source/torchdata.stateful_dataloader.rst

@@ -0,0 +1,13 @@
+:tocdepth: 3
+
+Stateful DataLoader
+===================
+
+.. automodule:: torchdata.stateful_dataloader
+
+StatefulDataLoader is a drop-in replacement for `torch.utils.data.DataLoader <https://pytorch.org/docs/stable/data.html#torch.utils.data.DataLoader>`_ which offers ``state_dict`` / ``load_state_dict`` methods for handling mid-epoch checkpointing which operate on the previous/next iterator requested from the dataloader (resp.).
+
+By default, the state includes the number of batches yielded and uses this to naively fast-forward the sampler (map-style) or the dataset (iterable-style). However if the sampler and/or dataset include ``state_dict`` / ``load_state_dict`` methods, then it will call them during its own ``state_dict`` / ``load_state_dict`` calls. Under the hood, :class:`StatefulDataLoader` handles aggregation and distribution of state across multiprocess workers (but not across ranks).
+
+.. autoclass:: StatefulDataLoader
+    :members:

torchdata/stateful_dataloader/stateful_dataloader.py

@@ -92,13 +92,12 @@
 
 class StatefulDataLoader(DataLoader[_T_co]):
     r"""
-    This is a drop in replacement for :class:`~torch.utils.data.DataLoader`
+    This is a drop in replacement for ``torch.utils.data.DataLoader``
     that implements state_dict and load_state_dict methods, enabling mid-epoch
     checkpointing.
 
-    All arguments are identical to :class:`~torch.utils.data.DataLoader`, with
-    a new kwarg: `snapshot_every_n_steps: Optional[int] = `.
-    See :py:mod:`torch.utils.data` documentation page for more details.
+    All arguments are identical to ``torch.utils.data.DataLoader``, with
+    a new kwarg: ``snapshot_every_n_steps``.
 
     Args:
         dataset (Dataset): dataset from which to load the data.
@@ -148,11 +147,13 @@ class StatefulDataLoader(DataLoader[_T_co]):
             maintain the workers `Dataset` instances alive. (default: ``False``)
         pin_memory_device (str, optional): the device to :attr:`pin_memory` to if ``pin_memory`` is
             ``True``.
+        snapshot_every_n_steps (int, optional): Defines how often the state is
+            transferred from the dataloader workers to the dataloader. By default, it is set to ``1``, i.e., state is transferred every step. If the state is large, this value can be increased (and ideally set to the frequency of training checkpointing) to reduce the overhead of transferring state every step.
 
 
     .. warning:: If the ``spawn`` start method is used, :attr:`worker_init_fn`
                  cannot be an unpicklable object, e.g., a lambda function. See
-                 :ref:`multiprocessing-best-practices` on more details related
+                 `multiprocessing-best-practices <https://pytorch.org/docs/stable/notes/multiprocessing.html#multiprocessing-best-practices>`_ on more details related
                  to multiprocessing in PyTorch.
 
     .. warning:: ``len(dataloader)`` heuristic is based on the length of the sampler used.
@@ -169,12 +170,12 @@ class StatefulDataLoader(DataLoader[_T_co]):
                  dropped when :attr:`drop_last` is set. Unfortunately, PyTorch can not detect such
                  cases in general.
 
-                 See `Dataset Types`_ for more details on these two types of datasets and how
+                 See `Dataset Types <https://pytorch.org/docs/stable/data.html>`_ for more details on these two types of datasets and how
                  :class:`~torch.utils.data.IterableDataset` interacts with
-                 `Multi-process data loading`_.
+                 `Multi-process data loading <https://pytorch.org/docs/stable/data.html#multi-process-data-loading>`_.
 
-    .. warning:: See :ref:`reproducibility`, and :ref:`dataloader-workers-random-seed`, and
-                 :ref:`data-loading-randomness` notes for random seed related questions.
+    .. warning:: See `Reproducibility <https://pytorch.org/docs/stable/notes/randomness.html#reproducibility>`_, and `Dataloader-workers-random-seed <https://pytorch.org/docs/stable/notes/faq.html#dataloader-workers-random-seed>`_, and
+                 `Data-loading-randomness <https://pytorch.org/docs/stable/data.html#data-loading-randomness>`_ notes for random seed related questions.
 
     .. _multiprocessing context:
         https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods