Add Loading nn.Module from checkpoint tutorial (non ghstack) (#2579)

* Add Module load_state_dict tutorial
---------
Co-authored-by: Svetlana Karslioglu <svekars@fb.com>

Author: mikaylagawarecki

recipes_source/recipes/module_load_state_dict_tips.py

@@ -0,0 +1,166 @@
+"""
+
+Tips for Loading an ``nn.Module`` from a Checkpoint
+===================================================
+
+If you're loading a checkpoint and want to reduce compute and memory as much as possible,
+this tutorial shares some recommended practices. In particular, we will discuss
+
+1.  The ``mmap`` keyword argument on ``torch.load``
+2.  The ``torch.device()`` context manager
+3.  The ``assign`` keyword argument on ``nn.Module.load_state_dict()``
+
+.. note::
+   This recipe requires PyTorch 2.1.0 or later.
+"""
+
+
+###############################################################################
+# Let us consider a simple ``nn.Module`` that contains a list of Linear layers:
+import torch
+from torch import nn
+import time
+
+class SomeModule(torch.nn.Module):
+    def __init__(self, size):
+        super().__init__()
+        self.linears = nn.ModuleList([nn.Linear(size, size) for i in range(10)])
+
+    def forward(self, x):
+        return self.linears(x)
+
+
+m = SomeModule(1000)
+torch.save(m.state_dict(), 'checkpoint.pth')
+
+#################################################################################
+# The following snippet demonstrates the use of the the ``mmap`` keyword argument
+# to ``torch.load``, the ``torch.device()`` context manager and the ``assign``
+# keyword argument to ``nn.Module.load_state_dict()``.
+
+state_dict = torch.load('checkpoint.pth', mmap=True)
+with torch.device('meta'):
+  meta_m = SomeModule(1000)
+meta_m.load_state_dict(state_dict, assign=True)
+
+#############################################################################
+# Compare the snippet below to the one above:
+
+state_dict = torch.load('checkpoint.pth')
+m = SomeModule(1000)
+m.load_state_dict(state_dict)
+
+#############################################################################
+# The second example does not use any of the features listed above and will be
+# less compute and memory efficient for loading a checkpoint. In the following
+# sections, we will discuss each of the features in further detail.
+
+#####################################################################################
+# Using ``torch.load(mmap=True)``
+# -------------------------------
+# First, let us consider what happens when we load the checkpoint with ``torch.load``.
+# When we save a checkpoint with ``torch.save``, tensor storages are tagged with the device they are
+# saved on. With ``torch.load``, tensor storages will be loaded to the device
+# they were tagged with (unless this behavior is overridden using the
+# ``map_location`` flag). For ease of explanation, let us assume that the tensors
+# were saved on CPU. This means that on the first line all tensor storages will be
+# loaded into CPU RAM, which can be undesirable when:
+#
+# * CPU RAM is smaller than the size of the checkpoint.
+# * Waiting for the entire checkpoint to be loaded into RAM before performing, for example, some per-tensor processing.
+
+start_time = time.time()
+state_dict = torch.load('checkpoint.pth')
+end_time = time.time()
+print(f"loading time without mmap={end_time - start_time}")
+
+#################################################################################
+# The ``mmap`` keyword argument to ``torch.load`` attempts to solve the above two
+# problems. As its name implies, the ``mmap`` keyword argument to ``torch.load``
+# makes use of an `mmap call <https://man7.org/linux/man-pages/man2/mmap.2.html>`_
+# which maps a file on disk into virtual memory and lets the OS handle loading and
+# unloading into physical memory automatically. When this flag is passed, tensor
+# storages will be memory-mapped.
+
+start_time = time.time()
+state_dict = torch.load('checkpoint.pth', mmap=True)
+end_time = time.time()
+print(f"loading time with mmap={end_time - start_time}")
+
+######################################################################################
+# As mentioned above, one can use this argument to do per-tensor processing on a
+# checkpoint without loading all tensor storages into CPU memory upfront. For example:
+def my_special_routine(t, device):
+    # this could be a much fancier operation
+    return t.to(dtype=torch.bfloat16, device=device)
+
+def my_processing_function(key, device):
+    t = state_dict[key]
+    processed_t = my_special_routine(t, device)
+    del t
+    state_dict[key] = processed_t
+
+for key in state_dict.keys():
+    device = torch.device('cuda')
+    my_processing_function(key, device)
+
+##################################################
+# Using ``torch.device('meta')``
+# ------------------------------
+# Next, let's consider the creation of the module.
+m = SomeModule(1000)
+
+#######################################################################################################
+# This allocates memory for all parameters/buffers and initializes them per
+# the default initialization schemes defined in ``SomeModule.__init__()``, which
+# is wasteful when we want to load a checkpoint for the following reasons:
+#
+# * The result of the initialization kernels will be overwritten by ``load_state_dict()`` without ever being used, so
+#   initialization is wasteful.
+# * We are allocating memory for these parameters/buffers in RAM while ``torch.load`` of the saved state dictionary also
+#   allocates memory in RAM for the parameters/buffers in the checkpoint.
+#
+# In order to solve these two problems, we can use the ``torch.device()``
+# context manager with ``device='meta'`` when we instantiate the ``nn.Module()``.
+#
+# The `torch.device() <https://pytorch.org/docs/main/tensor_attributes.html#torch-device>`_
+# context manager makes sure that factory calls will be performed as if they
+# were passed the specified ``device`` as an argument. Tensors on ``torch.device('meta')`` do not
+# carry data. However, they possess all other metadata a tensor carries such as ``.size()``, ``.stride()``,
+# ``.requires_grad``, and others.
+with torch.device('meta'):
+  new_m = SomeModule(1000)
+
+########################################################
+# Using ``load_state_dict(assign=True)``
+# --------------------------------------
+# Next, we consider the loading of the state dictionary.
+
+m.load_state_dict(state_dict)
+
+######################################################################################
+# ``nn.Module.load_state_dict()`` is usually implemented via an in-place
+# ``param_in_model.copy_(param_in_state_dict)``. This means that the parameter/buffer
+# with the corresponding key in the state dictionary is copied into the
+# parameter/buffer in the ``nn.Module``.
+#
+# However, an in-place copy into a tensor on the ``meta`` device is a no-op.
+# In order to avoid this, we can pass the ``assign=True`` keyword argument to
+# ``load_state_dict()``.
+#
+# A caveat here is that since optimizers hold a reference to
+# ``nn.Module.parameters()``, the optimizer must be initialized after the module
+# is loaded from state dict if ``assign=True`` is passed.
+
+new_m.load_state_dict(state_dict, assign=True)
+# This MUST be done AFTER the load_state_dict with assign.
+opt = torch.optim.SGD(new_m.parameters(), lr=1e-3)
+
+###############################################################################
+# Conclusion
+# -------------
+#
+# To recap, in this tutorial we learned about ``torch.load(mmap=True)``, the
+# ``torch.device()`` context manager with ``device=meta``, and
+# ``nn.Module.load_state_dict(assign=True)`` as well as how these tools could
+# be used to aid when loading a model from a checkpoint.

recipes_source/recipes_index.rst

@@ -137,6 +137,13 @@ Recipes are bite-sized, actionable examples of how to use specific PyTorch featu
    :link: ../recipes/recipes/reasoning_about_shapes.html
    :tags: Basics
 
+.. customcarditem::
+   :header: Tips for Loading an nn.Module from a Checkpoint
+   :card_description: Learn tips for loading an nn.Module from a checkpoint.
+   :image: ../_static/img/thumbnails/cropped/generic-pytorch-logo.png
+   :link: ../recipes/recipes/module_load_state_dict_tips.html
+   :tags: Basics
+
 .. Interpretability
 
 .. customcarditem::