# Glossary¶

ANN
Artificial Neural Network, often abbreviated as NN.
backend
A component that can execute computations.
bridge
A component of nGraph that acts as a backend for a framework, allowing the framework to define and execute computations.
data-flow graph
Data-flow graphs are used to implement deep learning models. In a data-flow graph, nodes represent operations on data and edges represent data flowing between those operations.
export
The serialized version of a trained model that can be passed to one of the nGraph backends for computation.
framework
Frameworks provide expressive user-facing APIs for constructing, training, validating, and deploying DL/ML models: TensorFlow*, PaddlePaddle*, MXNet*, PyTorch*, and Caffe* are all examples of well-known frameworks.
function graph
The Intel nGraph Library uses a function graph to represent an op’s parameters and results.
fusion
Fusion is the fusing, combining, merging, collapsing, or refactoring of a graph’s functional operations (ops) into one or more of nGraph’s core ops.
LSTM
LSTM is an acronym for “Long Short-Term Memory”. LSTMs extend on the traditional RNN by providing a number of ways to “forget” the memory of the previous time step via a set of learnable gates. These gates help avoid the problem of exploding or vanishing gradients that occur in the traditional RNN.
model description
A description of a program’s fundamental operations that are used by a framework to generate inputs for computation.
NN
NN is an acronym for “Neural Network”. NN models are used to simulate possible combinations of binary logic processing and multi-layer (multi-dimensional) paths through which a data-flow graph may be mapped or computed. A NN does not have centralized storage; rather, a NN manifests as information stored as patterns throughout the network structure. NNs may be Recurrent (feedback loop) or Nonrecurrent (feed-forward) with regard to the network vector.
op
An op represents an operation. Ops are stateless and have zero or more inputs and zero or more outputs. Some ops have additional constant attributes. Every output of an op corresponds to a tensor and has an element type and a shape. The element types and shapes of the outputs of an op are determined by the inputs and attributes of the op.
parameter
In the context of a function graph, a “parameter” refers to what “stands in” for an argument in an op definition.
quantization
Quantization refers to the conversion of numerical data into a lower-precision representation. Quantization is often used in deep learning to reduce the time and energy needed to perform computations by reducing the size of data transfers and the number of steps needed to perform a computation. This improvement in speed and energy usage comes at a cost in terms of numerical accuracy, but deep learning models are often able to function well in spite of this reduced accuracy.
RANN
Recurrent Artificial Neural Network, often abbreviated as RNN.
result
In the context of a function graph, the term “result” refers to what stands in for the returned value.
RNN

A Recurrent Neural Network is a variety of NN where output nodes from a layer on a data-flow graph have loopback to nodes that comprise an earlier layer. Since the RNN has no “centralized” storage, this loopback is the means by which the ANN can “learn” or be trained. There are several sub-categories of RNNs. The traditional RNN looks like:

$$s_t = tanh(dot(W,x_{t-1}) + dot(U, s_{t-1})$$

where $$x$$ is the input data, $$s$$ is the memory, and output is $$o_t = softmax(dot(V, s_t))$$. Tanh, Dot, and Softmax are all nGraph core Ops.

SGD
Stochastic Gradient Descent, also known as incremental gradient descent, is an iterative method for optimizing a differentiable objective function.
shape
The shape of a tensor is a tuple of non-negative integers that represents an exclusive upper bound for coordinate values.
shared pointer

The C++ standard template library has the template std::shared_ptr<X>. A shared pointer is used like an X* pointer, but maintains a reference count to the underlying object. Each new shared pointer to the object increases the count. When a shared pointer goes out of scope, the reference count is decremented, and, when the count reaches 0, the underlying object is deleted. The function template std::make_shared<X>(...) can be used similarly to new X(...), except it returns a std::shared_ptr<X> instead of an X*.

If there is a chain of shared pointers from an object back to itself, every object in the chain is referenced, so the reference counts will never reach 0 and the objects will never be deleted.

If a referenced b and b wanted to track all references to itself and shared pointers were used both directions, there would be a chain of pointers form a to itself. We avoid this by using shared pointers in only one direction, and raw pointers for the inverse direction. std::enabled_shared_from_this is a class template that defines a method shared_from_this that provides a shared pointer from a raw pointer.

nGraph makes use of shared pointers for objects whose lifetime is hard to determine when they are allocated.

step
An abstract “action” that produces zero or more tensor outputs from zero or more tensor inputs. Steps correspond to ops that connect nodes.
tensors

Tensors are maps from coordinates to scalar values, all of the same type, called the element type of the tensor.

Tensorview
The interface backends implement for tensor use. When there are no more references to the tensor view, it will be freed when convenient for the backend.
validated
To provide optimizations with nGraph, we first confirm that a given workload is “validated” as being functional; that is, we can successfully load its serialized graph as an nGraph function graph