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Script and Optimize for Mobile Recipe

This recipe demonstrates how to convert a PyTorch model to TorchScript which can run in a high-performance C++ environment such as iOS and Android, and how to optimize the converted TorchScript model for mobile deployment.

Introduction

After a PyTorch model is trained and optionally but preferably quantized (see Quantization Recipe for more details), one essential step before the model can be used in iOS and Android apps is to convert the Python-dependent model to TorchScript, which can then further be optimized for mobile apps. Conversion to TorchScript can be as simple as a single call, or as complicated as changing the original model in many different places.

Pre-requisites

PyTorch 1.6.0 or 1.7.0

Conversion to TorchScript

There are two basic ways to convert a PyTorch model to TorchScript, using trace and script. Mixing trace and script may also be needed in some cases - see here for more information.

Use the trace Method

To use the trace method on a model, an example or dummy input for the model needs to be specified, the actual input size needs to be the same as the example input size, and the model definition cannot have control flow such as if or for. The reason for these constraints is that running trace on a model with an example input simply calls the model’s forward method with the input and all operations executed in the model layers are recorded, creating the trace of the model.

import torch

dummy_input = torch.rand(1, 3, 224, 224)
torchscript_model = torch.jit.trace(model_quantized, dummy_input)

Use the script Method

For the example above, calling script below makes no difference:

torchscript_model = torch.jit.script(model_quantized)

But if a model has some flow control, then trace won’t correctly record all the possible traces. Take some code snippet of an example model definition from here for example:

import torch

class MyDecisionGate(torch.nn.Module):
    def forward(self, x):
        if x.sum() > 0:
            return x
        else:
            return -x

x = torch.rand(3, 4)
traced_cell = torch.jit.trace(MyDecisionGate(), x)
print(traced_cell.code)

The code above will output:

  TracerWarning: Converting a tensor to a Python boolean might cause the trace to be incorrect. We can''t record the data flow of Python values, so this value will be treated as a constant in the future. This means that the trace might not generalize to other inputs!

  if x.sum() > 0:
def forward(self,
    x: Tensor) -> Tensor:
  return x

Note that “the trace might not generalize to other inputs” warning above means that if the model has any kind of data-dependent control flow, trace is not the right answer. But if we replace the last two lines of the Python code snippet above (before the code output) with:

scripted_cell = torch.jit.script(MyDecisionGate())
print(scripted_cell.code)

The scripted model as shown by the print result below will be covering all possible inputs, thus generalizing to other inputs:

def forward(self,
    x: Tensor) -> Tensor:
  _0 = bool(torch.gt(torch.sum(x, dtype=None), 0))
  if _0:
    _1 = x
  else:
    _1 = torch.neg(x)
  return _1

This is another example of using trace and script - it converts the model trained in the PyTorch tutorial NLP FROM SCRATCH: TRANSLATION WITH A SEQUENCE TO SEQUENCE NETWORK AND ATTENTION:

encoder = EncoderRNN(input_lang.n_words, hidden_size)
decoder = AttnDecoderRNN(hidden_size, output_lang.n_words)

# method 1: using trace with example inputs

encoder_input=torch.tensor([1])
encoder_hidden=torch.zeros(1, 1, hidden_size)

decoder_input1=torch.tensor([[0]])
decoder_input2=torch.zeros(1, 1, hidden_size)
decoder_input3=torch.zeros(MAX_LENGTH, hidden_size)

traced_encoder = torch.jit.trace(encoder, (encoder_input, encoder_hidden))
traced_decoder = torch.jit.trace(decoder, (decoder_input1, decoder_input2, decoder_input3))

# method 2: using script

scripted_encoder = torch.jit.script(encoder)
scripted_decoder = torch.jit.script(decoder)

So is it true that one can simply always use the script call and the model is converted to TorchScript? The answer is no, because TorchScript is actually a subset of Python and to make script work, the PyTorch model definition must only use the language features of that TorchScript subset of Python. TorchScript Language Reference covers all the details of what is supported in TorchScript. Below we will describe some of the common errors when using the script method.

Fix Common Errors When Using the script Method

If you apply the script method to a non-trivial model, chances are you may encounter several types of errors. Check out this tutorial for a complete example of converting a chatbot model to TorchScript. But follow the steps below to fix common errors when you run the script method:

1. RuntimeError attribute lookup is not defined on python value of type

For this error, pass the value of the model as a parameter in the constructor. This is because when calling script on a model that accepts another model as a parameter, the model passed is actually of type TracedModule or ScriptModule, not of type Module, making the the model attribute not defined when scripting.

For example, the LuongAttnDecoderRNN module in the tutorial above has an attribute n_layers, and the GreedySearchDecoder module refers to the n_layers attribute of a decoder instance of the LuongAttnDecoderRNN module, so in order to make script work, the GreedySearchDecoder module’s constructor needs to be changed from:

def __init__(self, encoder, decoder):

to:

def __init__(self, encoder, decoder, decoder_n_layers):
  ...
  self._decoder_n_layers = decoder_n_layers

and the GreedySearchDecoder’s forward method needs to refer self._decoder_n_layers instead of decoder.n_layers.

2. RuntimeError python value of type ‘…’ cannot be used as a value.

The complete error message for this one continues with Perhaps it is a closed over global variable? If so, please consider passing it in as an argument or use a local variable instead., store global variables’ values as attributes in the model constructor (there’s no need to add them to a special list called __constants__). The reason is that global values can be used conveniently in normal model training and inference, but the global values are not accessible during the scripting.

For example, device and SOS_token are global variables, and to make script work, they need to be added to the GreedySearchDecoder’s constructor:

self._device = device
self._SOS_token = SOS_token

and referred to as self._device and self._SOS_token instead of device and SOS_token in the GreedySearchDecoder’s forward method.

3. RuntimeError all inputs of range must be ‘…’, found Tensor (inferred) in argument

The error message continues with: add type definitions for each of the module’s forward method arguments. Because all parameters to a TorchScript function are of the `torch.Tensor type by default, you need to specifically declare the type for each parameter that is not of type ‘Tensor’. For a complete list of TorchScript-supported types, see here.

For example, the GreedySearchDecoder’s forward method signature needs to be changed from:

def forward(self, input_seq, input_length, max_length):

to:

def forward(self, input_seq, input_length, max_length : int):

After using the trace or script method above, and fixing possible errors, you should have a TorchScript model ready to be optimized for mobile.

Optimize a TorchScript Model

Simply run the following code snippet to optimize a TorchScript model generated with the trace and/or script method:

from torch.utils.mobile_optimizer import optimize_for_mobile
optimized_torchscript_model = optimize_for_mobile(torchscript_model)

The optimized model can then be saved and deployed in mobile apps:

optimized_torchscript_model.save("optimized_torchscript_model.pth")

By default, optimize_for_mobile will perform the following types of optimizations:

  • Conv2D and BatchNorm fusion which folds Conv2d-BatchNorm2d into Conv2d;
  • Insert and fold prepacked ops which rewrites the model graph to replace 2D convolutions and linear ops with their prepacked counterparts.
  • ReLU and hardtanh fusion which rewrites graph by finding ReLU/hardtanh ops and fuses them together.
  • Dropout removal which removes dropout nodes from this module when training is false.

Learn More

  1. The official TorchScript Language Reference.
  2. The torch.utils.mobile_optimizer API documentation.

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