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# Architecture

Brevitas is organized around a few different concepts listed below.

## Functions

Functions include both Autograd functions and TorchScript functions and can be found under brevitas.function.

Autograd functions implement straight-through estimators. All Autograd functions are implemented both in Python, found under brevitas.ops.autograd_ste_ops, and C++, found under brevitas/csrc. This is because to date Python Autograd functions cannot be compiled by Pytorch’s JIT compiler, but C++ Autograd functions can. Compiling the C++ extension is performed at runtime in order to simplify packaging and distribution (only a single .cpp source file is distributed). This requires the user to have an appropriate C++ compiler, so a Python implemention is provided as fallback.

Because of this, as long as BREVITAS_JIT=0 (which it is by default), the C++ backend is not loaded. Wrapping and switching between the two implementations happens in brevitas.function.ops_ste.

TorchScript functions implements various bits and pieces used commonly across Brevitas that are required to be JIT-compilable. They can be found under brevitas.function.ops and brevitas.function.shape.

## Core ScriptModules

The algorithmic core of Brevitas can be found under brevitas.core. The core implements all the various building blocks required to assemble an affine quantizer. All building blocks are implemented as a ScriptModule in the old-style TorchScript scriping API (i.e. inheriting from torch.jit.ScriptModule). Many of the functions described in the section above are called from within one of the core ScriptModules. Any module within brevitas.core is required to be a ScriptModule compatible with Pytorch 1.3.0 JIT compiler, with most building blocks being compatible with Pytorch 1.1.0 JIT compiler. Implementing everything in TorchScript enables compute and memory optimizations at training time, which for more complicated quantization pipelines can be quite significant, thus reducing the intrinsic training-time cost of quantization-aware training.

Adhering to the restriction imposed by TorchScript pose a challange in terms of how to achieve flexibility while minimizing redundancy. Because the flavour of TorchScript adopted by Brevitas does not allow for inheritance, Brevitas’ core is highly biased towards leveraging composition. In particular, the implementation favours inversion of control through dependency injection (DI), whether manually or through the DI library dependencies (as explained in the Injectors section below.)

## Injectors and Quantizers

Auto-wiring dependency injection (DI) with dependencies is the machinery at the heart of Brevitas. If you have ever used fixtures in pytest, you already know (high-level) how auto-wiring DI works. In the case of dependencies, the idea is that the objects to be instantiated and wired together are declared as attributes of an Injector class. The driving mechanism behind the auto-wiring process is to match the name of an attribute of an Injector with them name of arguments required to build other attributes of the same Injectors. When applied to Brevitas then, the idea is to throw together a bunch of components appropriately chosen from brevitas.core as attributes of an Injector, and have the DI library automatically assemble them together. A quantizer then is an Injector with a tensor_quant attribute. A tensor_quant object is expected to be a torch Module (either a torch.nn.Module or a torch.jit.ScriptModule) that takes in as input a torch.Tensor to quantize, and return as output a tuple containing four torch.Tensor, representing respectively the output quantized tensor in dequantized format, its scale factor, its zero-point, and it’s bit-width.

Injectors are a powerful way to express quantizers: being standard Python classes, they lend themselves to both inheritance and composition (through multiple inheritance in a mixin style). That means for example that a new quantizer can be defined simply by inheriting from an existing one and overriding some of its attributes, or by inheriting from multiple smaller Injector that declare only some of the components required to assemble a quantizer. Pre-built quantizers can be found under brevitas.quant. Specifically, brevitas.quant.shifted_scaled_int holds quantizers with zero-point != 0. and floating-point scale factors, brevitas.quant.scaled_int holds quantizers with zero-point == 0. and floating-point scale factors, and brevitas.quant.fixed_point holds fixed-point, i.e. quantizers with zero-point == 0 and power-of-two scale factors. Pre-built quantizers are assembled together from smaller Injectors that can be found in brevitas.quant.base.

Brevitas depends on a specific older version of dependencies (v2.0.1) that plays well with ScriptModules, and it extends it with an ExtendedInjector class that provides support for some additional features. Specifically, ExtendedInjector allows to dynamically declare an object to auto-wire by returning a class from a @value function. This style of syntax allows to create intermediate abstractions between the brevitas.core components and the brevitas.quant quantizers, as explained in the section below.

## Enums, Shapes and Solvers

Trying to navigate all the various components in brevitas.core that can be assembled together in a quantizer can be confusing. The lack a of clear object hierarchy between various ScriptModules means that it’s not obvious which components adhere to a certain interface. Additionally, while some components might fit together from a purely software perspective, they might not work together from a machine learning perspective. In order to provide a simplified interface that abstracts away some of these details, Brevitas provides various Enum on top of the components found in brevitas.core.

Some examples of enums are: QuantType specifies the type of quantization, e.g. QuantType.BINARY for binary quantization. ScalingImplType, for specifying the type (algorithmic-wise) of scaling, e.g. ScalingImplType.PARAMETER_FROM_STATS to specify that the scale factor should be a learned torch.nn.Parameter initialized from statistics of the tensor to quantize. Enums can currently be found under brevitas.inject.enum.

Depending on the kind of tensor to quantize, say weights vs activations, the same enum value is gonna translate to different brevitas.core components. So for example QuantType.BINARY translates to brevitas.core.quant.BinaryQuant for weights and to brevitas.core.quant.ClampedBinaryQuant for activations. This way enums allows to declare quantizers while abstract away from the specifics of a target tensor. In general there can be a 1-to-1, many-to-1 or many-to-many relationship between enums and brevitas.core components and their hyperparameters.

The translation between enums and brevitas.core is performed by a solver, which can be found under brevitas.quant.solver.Solvers are really just an ExtendedInjector that take advantage of the extended syntax of Injectors implemented in Brevitas to translate e.g. quant_type = QuantType.BINARY to tensor_quant = BinaryQuant within the scope of a quantizer at dependency-injection time. That means that solvers are as composable as quantizers are, so for example to solve enums against weight quantization, it’s enough to create a quantizer that inherits from brevitas.quant.solver.weight.WeightQuantSolver, which is itself just the collection of various solvers for individual tasks. Looking at the quantizers found under brevitas.quant, it can be seen that for the most part they actually specify enums rather than directly brevitas.core components. Then enums are then solved to different core components depending on which solver is applied. This is meant to provide a blueprint for users to understand which enums are supported and how they go together.

A similar mechanism applies when for example the directive scaling_per_output_channel=True is specified. In the case of ScalingImplType.PARAMETER_FROM_STATS, that means that a torch.nn.Parameter with size the number of channels of the tensor to quantize has to be allocated. Because TorchScript does not allow for much dynamic behaviour, the shape of the parameter has to be known at dependency-injection time. Rather than forcing the user to specify the appropriate shape in a quantizer, Brevitas is capable of inferring it from the nn.Module whose weights the quantizer is applied to. This again happens by means of a solver included as part of WeightQuantSolver.

Thanks to how dependencies works, solvers are invoked only whenever their output is actually required to build an attribute of an ExtendedInjector, which in the case of Brevitas is tensor_quant. Additionally, by specifying a solver as last in the list of classes from which a quantizer inherits from, it’s always possible to override its behaviour and directly declare its output. So for example it’s possible to directly declare tensor_quant = BinaryQuant instead of quant_type = QuantType.BINARY even when WeightQuantSolver is applied to a quantizer. This allows more advanced users to mix-and-match enums with custom components. Finally, it’s always possibly to just not apply a solver to a quantizer and simply declare everything manually.

## QuantTensor

A QuantTensor is a custom data structure for representing a uniform, affine quantized tensor. It can be found under brevitas.quant_tensor. It can be valid or non-valid. A non-valid QuantTensor is simply a wrapper around a torch.Tensor that had been previously quantized and is now in dequantized format. The QuantTensor is marked as non-valid because it doesn’t carry enough information to derive its quantized representation back. A valid QuantTensor carries scale, zero-point, bit-width, sign, and whether it was generated in training or inference mode.

The arithmetic of QuantTensors implments a generalized version of fixed-point arithmetic, with the main assumption being that only two QuantTensor with the same scale factor can be summed together. This constrain is enforced when the QuantTensors involved in a sum have been generated in inference mode, but it’s not enforced in training mode. This is because when dealing with e.g. ScalingImplType.PARAMETER_FROM_STATS, the activation tensors in a residual topology can have different scale factors along the skip connection at training time, but not at inference time.

## Proxies

A proxy is a nn.Module that wraps a quantizer. Proxies can be found under brevitas.proxy. Proxies are specialized w.r.t. the kind of tensor they quantize, such as weights, biases, or activations.

The main responsability of a proxy is to make sure a QuantTensor is returned as output of quantization, which wouldn’t be possible in TorchScript. Additionally, it has to make sure a quantizer is re-initialized any time it is necessary.

For example, when performing ScalingImplType.PARAMETER_FROM_STATS scaling on a weight tensor, statistics of the weight tensor are computed at dependency-injection time and used to initialize a learned parameter. However, it’s not possible to know a-priori whether a pretrained floating-point state-dict will be later on loaded on top of a quantized model definition. In that case, any initialization logic that depends on the state-dict of the model that is being quantized has to be recomputed. Injectors invoked by proxies on state_dict changes allow to do so automatically, providing a mechanism to reconcile the inherent rigidity of TorchScript with the typical define-by-run execution of Pytorch models.

Proxies also allow to support more complex quantization scenarios, such as when the same quantizer has to be shared between different layers. A typical situation where that happens is when the output of multiple branches of a residual topology are summed together at high-precision, without requantizing first. In that case, the weight tensors of the layers that feed into the accumulator need to have the same scale factor. That can be accomplished by declaring a single WeightQuantProxy that is shared among multiple layers. What happens is that - for example for ScalingImplType.STATS, ScalingImplType.AFFINE_STATS or ScalingImplType.PARAMETER_FROM_STATS - the scale factor is computed as a statistics of the concatenation of the weight tensors to be quantized. Because it can’t be known a-priori to between how many layers the same WeightQuantProxy is shared, again every time a WeightQuantProxy starts tracking a new weight tensor, the underlying quantizer has to be re-initialized.

## Quant Layers and Mixins

A QuantLayer is the quantized variant of a torch.nn layer, and can be found under brevitas.nn. Typically a QuantLayer inherits from both its floating-point variant (e.g. QuantConv2d inherits from Conv2d), plus a serie of mixins, each responsibile for instantiating a proxy within the QuantLayer. A mixin is more specialized than a proxy, so for example both QuantInputMixin and QuantOutputMixin instantiate an activation quantization proxy. So-called _QuantWBIOL_ layers (such as QuantConv2d) inherit from QuantWeightMixin, QuantBiasMixin, QuantInputMixin and QuantOutputMixin. That means that they can quantize respectively weight, bias, input and output. Quantizers can be passed to a QuantWBIOL layer by setting respectively quant_weight=, quant_bias=, quant_input= and quant_output=. If an ExtendedInjector is passed it, a proxy will be allocated by the mixin to deal with its initialization. Otherwise, if a proxy is passed in, it will be set as-is. Setting e.g. quant_weight=None will disable quantization for weights. A layer where quantization is disabled is supposed to act exactly like its floating-point counterpart, so QuantConv2d(…, weight_quant=None, bias_quant=None, input_quant=None, output_quant=None) behaves like a Conv2d layer.

Typically torch.nn layers expose a flat interface. To support a similar UX in Brevitas, QuantLayers support setting attributes in a quantizer by passing keyword arguments with an appropriate prefix. For QuantWBIOL layers, keyword arguments with prefix weight_ are passed to the weight_quant quantizer, bias_ to bias_quant, input_ to input_quant, and output_ to output_quant. For quantized activation layers, like QuantReLU, a prefix is not required, and keyword arguments are directly passed to output quantization. In case an ExtendedInjector is not set, e.g. if weight_quant=None, but additional keyword arguments are passed in, an empty ExtendedInjector is automatically allocated and keyword arguments are set as its attribute according to the their prefix. Keyword arguments have priority over pre-existing attribute of a quantizer, so passing a keyword argument is a way to override the attribute of a quantizer on an individual-layer level.

In many real-life scenarios, a user might want to first quantize only certain layers or certain parts of certain layers to perform some exploratory analysis in terms of accuracy. Correctness w.r.t. the specifics of a target hardware, if any, might not be a concern. To minimize friction with adoption then, Brevitas is designed to remain functional as much as possible under partially specified information. With the exception of TruncAvgPool2d, a QuantLayer is not expected to receive a QuantTensor as input (altough doing so enables more scenarios), nor it returns one by default (i.e. return_quant_tensor=False by default). Specifically, The output of a QuantLayer is always in de-quantized format (whether wrapped in a valid or non-valid QuantTensor or not). That means that QuantLayers can be easily mixed with standard torch.nn layers.

## Export

TODO

## FX graph tracing and transformations

TODO

## Losses

TODO