Cherry-pick : Add documentation for dynamo.compile backend (#2389) (#2416)

Signed-off-by: Dheeraj Peri <[email protected]>

Author: peri044

docsrc/index.rst

@@ -41,6 +41,7 @@ User Guide
 * :ref:`creating_a_ts_mod`
 * :ref:`getting_started_with_fx`
 * :ref:`torch_compile`
+* :ref:`dynamo_export`
 * :ref:`ptq`
 * :ref:`runtime`
 * :ref:`saving_models`
@@ -56,6 +57,7 @@ User Guide
    user_guide/creating_torchscript_module_in_python
    user_guide/getting_started_with_fx_path
    user_guide/torch_compile
+   user_guide/dynamo_export
    user_guide/ptq
    user_guide/runtime
    user_guide/saving_models

docsrc/user_guide/dynamo_export.rst

@@ -0,0 +1,82 @@
+.. _dynamo_export:
+
+Torch-TensorRT Dynamo Backend
+=============================================
+.. currentmodule:: torch_tensorrt.dynamo
+
+.. automodule:: torch_tensorrt.dynamo
+   :members:
+   :undoc-members:
+   :show-inheritance:
+   
+This guide presents Torch-TensorRT dynamo backend which optimizes Pytorch models 
+using TensorRT in an Ahead-Of-Time fashion. 
+
+Using the Dynamo backend 
+----------------------------------------
+Pytorch 2.1 introduced ``torch.export`` APIs which 
+can export graphs from Pytorch programs into ``ExportedProgram`` objects. Torch-TensorRT dynamo 
+backend compiles these ``ExportedProgram`` objects and optimizes them using TensorRT. Here's a simple 
+usage of the dynamo backend
+
+.. code-block:: python
+
+    import torch
+    import torch_tensorrt
+
+    model = MyModel().eval().cuda()
+    inputs = [torch.randn((1, 3, 224, 224), dtype=torch.float32).cuda()]
+    exp_program = torch.export.export(model, tuple(inputs))
+    trt_gm = torch_tensorrt.dynamo.compile(exp_program, inputs) # Output is a torch.fx.GraphModule
+    trt_gm(*inputs)
+
+.. note::  ``torch_tensorrt.dynamo.compile`` is the main API for users to interact with Torch-TensorRT dynamo backend. The input type of the model should be ``ExportedProgram`` (ideally the output of ``torch.export.export`` or ``torch_tensorrt.dynamo.trace`` (discussed in the section below)) and output type is a ``torch.fx.GraphModule`` object.
+
+Customizeable Settings
+----------------------
+
+There are lot of options for users to customize their settings for optimizing with TensorRT. 
+Some of the frequently used options are as follows: 
+
+* ``inputs`` - For static shapes, this can be a list of torch tensors or `torch_tensorrt.Input` objects. For dynamic shapes, this should be a list of ``torch_tensorrt.Input`` objects.
+* ``enabled_precisions`` - Set of precisions that TensorRT builder can use during optimization.
+* ``truncate_long_and_double`` - Truncates long and double values to int and floats respectively.
+* ``torch_executed_ops`` - Operators which are forced to be executed by Torch.
+* ``min_block_size`` - Minimum number of consecutive operators required to be executed as a TensorRT segment.
+
+The complete list of options can be found `here <https://github.com/pytorch/TensorRT/blob/123a486d6644a5bbeeec33e2f32257349acc0b8f/py/torch_tensorrt/dynamo/compile.py#L51-L77>`_
+
+.. note:: We do not support INT precision currently in Dynamo. Support for this currently exists in 
+our Torchscript IR. We plan to implement similar support for dynamo in our next release.
+
+Under the hood
+--------------
+
+Under the hood, ``torch_tensorrt.dynamo.compile`` performs the following on the graph.
+
+* Lowering - Applies lowering passes to add/remove operators for optimal conversion.
+* Partitioning - Partitions the graph into Pytorch and TensorRT segments based on the ``min_block_size`` and ``torch_executed_ops`` field.
+* Conversion - Pytorch ops get converted into TensorRT ops in this phase.
+* Optimization - Post conversion, we build the TensorRT engine and embed this inside the pytorch graph.
+
+Tracing
+-------
+
+``torch_tensorrt.dynamo.trace`` can be used to trace a Pytorch graphs and produce ``ExportedProgram``. 
+This internally performs some decompositions of operators for downstream optimization. 
+The ``ExportedProgram`` can then be used with ``torch_tensorrt.dynamo.compile`` API.
+If you have dynamic input shapes in your model, you can use this ``torch_tensorrt.dynamo.trace`` to export 
+the model with dynamic shapes. Alternatively, you can use ``torch.export`` `with constraints <https://pytorch.org/docs/stable/export.html#expressing-dynamism>`_ directly as well.
+
+.. code-block:: python
+
+    import torch
+    import torch_tensorrt
+
+    inputs = [torch_tensorrt.Input(min_shape=(1, 3, 224, 224),
+                                  opt_shape=(4, 3, 224, 224),
+                                  max_shape=(8, 3, 224, 224),
+                                  dtype=torch.float32)]
+    model = MyModel().eval()
+    exp_program = torch_tensorrt.dynamo.trace(model, inputs) 
+    

docsrc/user_guide/saving_models.rst

@@ -1,16 +1,25 @@
-.. _runtime:
+.. _saving_models:
 
 Saving models compiled with Torch-TensorRT
 ====================================
+.. currentmodule:: torch_tensorrt.dynamo
 
+.. automodule:: torch_tensorrt.dynamo
+   :members:
+   :undoc-members:
+   :show-inheritance:
+   
 Saving models compiled with Torch-TensorRT varies slightly with the `ir` that has been used for compilation.
 
-1) Dynamo IR
+Dynamo IR 
+-------------
 
 Starting with 2.1 release of Torch-TensorRT, we are switching the default compilation to be dynamo based.
 The output of `ir=dynamo` compilation is a `torch.fx.GraphModule` object. There are two ways to save these objects
 
 a) Converting to Torchscript
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 `torch.fx.GraphModule` objects cannot be serialized directly. Hence we use `torch.jit.trace` to convert this into a `ScriptModule` object which can be saved to disk. 
 The following code illustrates this approach. 
 
@@ -30,6 +39,8 @@ The following code illustrates this approach.
     model(inputs)
 
 b) ExportedProgram
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 `torch.export.ExportedProgram` is a new format introduced in Pytorch 2.1. After we compile a Pytorch module using Torch-TensorRT, the resultant 
 `torch.fx.GraphModule` along with additional metadata can be used to create `ExportedProgram` which can be saved and loaded from disk.
 
@@ -54,7 +65,9 @@ This is needed as `torch._export` serialization cannot handle serializing and de
 
 NOTE: This way of saving the models using `ExportedProgram` is experimental. Here is a known issue : https://github.com/pytorch/TensorRT/issues/2341
 
-2) Torchscript IR
+
+Torchscript IR
+-------------
 
   In Torch-TensorRT 1.X versions, the primary way to compile and run inference with Torch-TensorRT is using Torchscript IR.
   This behavior stays the same in 2.X versions as well.