Add custom class pt2 tutorial

advanced_source/custom_class_pt2.rst

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+Supporting Custom C++ Classes in PyTorch 2
+==========================================
+
+This tutorial is a follow-on to the
+:doc:`custom C++ classes <torch_script_custom_classes>` tutorial, and
+introduces additional steps that are needed to support custom C++ classes in
+PyTorch 2.
+
+Concretely, there are a few steps:
+
+1. Implement an ``__obj_flatten__`` method to the C++ custom class
+    implementation to allow us to inspect its states and guard the changes. The
+    method should return a tuple of tuple of attribute_name, value
+    (``tuple[tuple[str, value] * n]``).
+2. Register a python fake class using ``@torch._library.register_fake_class``
+    a. Implement “fake methods” of each of the class’s c++ methods, which should
+        have the same schema as the C++ implementation.
+    b. Additionally, implement an ``__obj_unflatten__`` classmethod in the Python
+        fake class to tell us how to create a fake class from the flattened
+        states returned by ``__obj_flatten__``.
+
+Here is a breakdown of the diff. Following the guide in
+:doc:`Extending TorchScript with Custom C++ Classes <torch_script_custom_classes>`,
+we can create a thread-safe tensor queue and build it.
+
+.. code-block:: cpp
+
+    // Thread-safe Tensor Queue
+    struct TensorQueue : torch::CustomClassHolder {
+    ...
+    private:
+    std::deque<at::Tensor> queue_;
+    std::mutex mutex_;
+    at::Tensor init_tensor_;
+    };
+    // The torch binding code
+    TORCH_LIBRARY(MyCustomClass, m) {
+    m.class_<TensorQueue>("TensorQueue")
+        .def(torch::init<at::Tensor>())
+        .def("push", &TensorQueue::push)
+        .def("pop", &TensorQueue::pop)
+        .def("top", &TensorQueue::top)
+        .def("size", &TensorQueue::size)
+        .def("clone_queue", &TensorQueue::clone_queue)
+        .def("get_raw_queue", &TensorQueue::get_raw_queue)
+        .def_pickle(
+            // __getstate__
+            [](const c10::intrusive_ptr<TensorQueue>& self)
+                -> c10::Dict<std::string, at::Tensor> {
+                return self->serialize();
+            },
+            // __setstate__
+            [](c10::Dict<std::string, at::Tensor> data)
+                -> c10::intrusive_ptr<TensorQueue> {
+                return c10::make_intrusive<TensorQueue>(std::move(data));
+            });
+    }
+
+**Step 1**: Add an ``__obj_flatten__`` method to the C++ custom class implementation:
+
+.. code-block:: cpp
+
+    // Thread-safe Tensor Queue
+    struct TensorQueue : torch::CustomClassHolder {
+    ...
+    std::tuple<std::tuple<std::string, std::vector<at::Tensor>>, std::tuple<std::string, at::Tensor>> __obj_flatten__() {
+        return std::tuple(std::tuple("queue", this->get_raw_queue()), std::tuple("init_tensor_", this->init_tensor_.clone()));
+    }
+    ...
+    }
+
+    TORCH_LIBRARY(MyCustomClass, m) {
+    m.class_<TensorQueue>("TensorQueue")
+        .def(torch::init<at::Tensor>())
+        .def("__obj_flatten__", &TensorQueue::__obj_flatten__)
+        ...
+    }
+
+**Step 2a**: Register a fake class in Python that implements each method.
+
+.. code-block:: python
+
+    # namespace::class_name
+    @torch._library.register_fake_class("MyCustomClass::TensorQueue")
+    class FakeTensorQueue:
+        def __init__(
+            self,
+            queue: List[torch.Tensor],
+            init_tensor_: torch.Tensor
+        ) -> None:
+            self.queue = queue
+            self.init_tensor_ = init_tensor_
+
+        def push(self, tensor: torch.Tensor) -> None:
+            self.queue.append(tensor)
+
+        def pop(self) -> torch.Tensor:
+            if len(self.queue) > 0:
+                return self.queue.pop(0)
+            return self.init_tensor_
+
+        def size(self) -> int:
+        return len(self.queue)
+
+**Step 2b**: Implement an ``__obj_unflatten__`` classmethod in Python.
+
+.. code-block:: python
+
+    # namespace::class_name
+    @torch._library.register_fake_class("MyCustomClass::TensorQueue")
+    class FakeTensorQueue:
+    ...
+        @classmethod
+        def __obj_unflatten__(cls, flattened_tq):
+            return cls(**dict(flattened_tq))
+
+    ...
+
+That’s it! Now we can create a module that uses this object and run it with ``torch.compile`` or ``torch.export``:
+
+.. code-block::python
+
+    import torch
+
+    torch.ops.load_library("//caffe2/test:test_torchbind_cpp_impl")
+    tq = torch.classes.MyCustomClass.TensorQueue(torch.empty(0).fill_(-1))
+
+    class Mod(torch.nn.Module):
+        def forward(self, tq, x):
+            tq.push(x.sin())
+            tq.push(x.cos())
+            poped_t = tq.pop()
+            assert torch.allclose(poped_t, x.sin())
+            return tq, poped_t
+
+    tq, poped_t = torch.compile(Mod(), backend="eager", fullgraph=True)(tq, torch.randn(2, 3))
+    assert tq.size() == 1
+
+    exported_program = torch.export.export(Mod(), (tq, torch.randn(2, 3),), strict=False)
+    exported_program.module()(tq, torch.randn(2, 3))
+
+We can also implement custom ops that take custom classes as inputs. For
+example, we could register a custom op ``for_each_add_(tq, tensor)``
+
+.. code-block:: cpp
+
+    struct TensorQueue : torch::CustomClassHolder {
+    ...
+    void for_each_add_(at::Tensor inc) {
+        for (auto& t : queue_) {
+        t.add_(inc);
+        }
+    }
+    ...
+    }
+
+
+    TORCH_LIBRARY_FRAGMENT(MyCustomClass, m) {
+    m.class_<TensorQueue>("TensorQueue")
+    .def("for_each_add_", &TensorQueue::for_each_add_);
+
+    m.def(
+        "for_each_add_(__torch__.torch.classes.MyCustomClass.TensorQueue foo, Tensor inc) -> ()");
+
+    }
+
+    void for_each_add_(c10::intrusive_ptr<TensorQueue> tq, at::Tensor inc) {
+    tq->for_each_add_(inc);
+    }
+
+    TORCH_LIBRARY_IMPL(MyCustomClass, CPU, m) {
+    m.impl("for_each_add_", for_each_add_);
+    }
+
+
+Since the fake class is implemented in python, we require the fake
+implementation of custom op must also be registered in python:
+
+.. code-block:: python
+
+    @torch.library.register_fake("MyCustomClass::for_each_add_")
+    def fake_for_each_add_(tq, inc):
+        tq.for_each_add_(inc)
+
+After re-compilation, we can export the custom op with:
+
+.. code-block:: python
+
+    class ForEachAdd(torch.nn.Module):
+        def forward(self, tq: torch.ScriptObject, a: torch.Tensor) -> torch.ScriptObject:
+            torch.ops.MyCustomClass.for_each_add_(tq, a)
+            return tq
+
+    mod = ForEachAdd()
+    tq = empty_tensor_queue()
+    qlen = 10
+    for i in range(qlen):
+    tq.push(torch.zeros(1))
+
+    ep = torch.export.export(mod, (tq, torch.ones(1)), strict=False)
+
+Why do we need to make a Fake Class?
+------------------------------------
+
+Tracing with real custom object has several major downsides:
+1. Operators on real objects can be time consuming e.g. the custom object
+    might be reading from the network or loading data from the disk.
+2. We don’t want to mutate the real custom object or create side-effects to the environment while tracing.
+3. It cannot support dynamic shapes.
+
+However, it may be difficult for users to write a fake class: the original class
+uses some third-party library that determines the output shape of the methods,
+or is complicated and written by others. Besides, users may not care about the
+limitations listed above. In this case, please reach out to us!