chore: update dynamo export doc

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

Author: peri044

docsrc/index.rst

@@ -40,6 +40,7 @@ User Guide
 ------------
 * :ref:`creating_a_ts_mod`
 * :ref:`getting_started_with_fx`
+* :ref:`dynamo_export`
 * :ref:`ptq`
 * :ref:`runtime`
 * :ref:`saving_models`
@@ -54,6 +55,7 @@ User Guide
 
    user_guide/creating_torchscript_module_in_python
    user_guide/getting_started_with_fx_path
+   user_guide/dynamo_export
    user_guide/ptq
    user_guide/runtime
    user_guide/saving_models

docsrc/user_guide/dynamo_export.rst

@@ -0,0 +1,72 @@
+.. _getting_started_with_dynamo:
+
+Torch-TensorRT (Dynamo) Backend
+========================
+This guide presents Torch-TensorRT dynamo backend which compiles Pytorch programs 
+into TensorRT engines through torch dynamo. Pytorch 2.1 introduced ``torch.export`` APIs which 
+can export graphs from Pytorch programs using torch dynamo. Torch-TensorRT dynamo 
+backend compiles these exported graphs 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(model, inputs)
+    trt_gm = torch_tensorrt.dynamo.compile(exp_program, inputs) # Output is a torch.fx.GraphModule
+    trt_gm(inputs)
+
+``torch_tensorrt.dynamo.compile`` is the main API for users to interact with Torch-TensorRT.
+The input type of the model should be ``ExportedProgram`` (ideally the output of torch.export) and output types is a ``torch.fx.GraphModule`` object.
+
+Customizations
+---------------------------------------------
+
+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 a) torch tensors or b) `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) 
+