docs: add autoload

Author: shink

_static/img/python_backend_autoload_impl.png

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+Out-of-tree extension autoloading in Python
+===========================================
+
+What is it?
+-----------
+
+The extension autoloading mechanism enables PyTorch to automatically
+load out-of-tree backend extensions without explicit import statements. This
+mechanism is very useful for users. On the one hand, it improves the user
+experience and enables users to adhere to the familiar PyTorch device
+programming model without needing to explicitly load or import device-specific
+extensions. On the other hand, it facilitates effortless
+adoption of existing PyTorch applications with zero-code changes on
+out-of-tree devices.
+
+Examples
+^^^^^^^^
+
+`habana_frameworks.torch`_ is a Python package that enables users to run
+PyTorch programs on Intel Gaudi via the PyTorch ``HPU`` device key.
+``import habana_frameworks.torch`` is no longer necessary after this mechanism
+is applied.
+
+.. _habana_frameworks.torch: https://docs.habana.ai/en/latest/PyTorch/Getting_Started_with_PyTorch_and_Gaudi/Getting_Started_with_PyTorch.html
+
+.. code-block:: diff
+
+    import torch
+    import torchvision.models as models
+    - import habana_frameworks.torch # <-- extra import
+    model = models.resnet50().eval().to(“hpu”)
+    input = torch.rand(128, 3, 224, 224).to(“hpu”)
+    output = model(input)
+
+`torch_npu`_ enables users to run PyTorch program on Huawei Ascend NPU, it
+leverages the ``PrivateUse1`` device key and exposes the device name
+as ``npu`` to the end users.
+``import torch_npu`` is also no longer needed after applying this mechanism.
+
+.. _torch_npu: https://github.com/Ascend/pytorch
+
+.. code-block:: diff
+
+    import torch
+    import torchvision.models as models
+    - import torch_npu # <-- extra import
+    model = models.resnet50().eval().to(“npu”)
+    input = torch.rand(128, 3, 224, 224).to(“npu”)
+    output = model(input)
+
+How it works
+------------
+
+.. image:: ../_static/img/python_backend_autoload_impl.png
+   :alt: Autoloading implementation
+   :align: center
+
+This mechanism is implemented based on Python's `entry_points`_ mechanism.
+We discover and load all of the specific entry points in ``torch/__init__.py``
+that are defined by out-of-tree extensions.
+
+.. _entry_points: https://packaging.python.org/en/latest/specifications/entry-points/
+
+How to apply this to out-of-tree extensions?
+--------------------------------------------
+
+For example, if you have a package named ``torch_foo`` and it includes the
+following in its ``__init__.py``:
+
+.. code-block:: python
+
+    def _autoload():
+        print("No need to import torch_foo anymore! You can run torch.foo.is_available() directly.")
+
+Then the only thing you need to do is add an entry point to your Python
+package.
+
+.. code-block:: python
+
+    setup(
+        name="torch_foo",
+        version="1.0",
+        entry_points={
+            'torch.backends': [
+                'torch_foo = torch_foo:_autoload',
+            ],
+        }
+    )
+
+Now the ``torch_foo`` module can be imported when running import torch.
+
+Conclusion
+----------
+
+This tutorial has guided you through the out-of-tree extension autoloading
+mechanism, including its usage and implementation.