v1.0.0-alpha

With this release we introduce breaking changes that bring significant
improvements to the project's structure, API and performance.

It would be difficult and confusing to list every single API change. Instead,
the following sections will broadly describe the most relevant changes,
arranged by topic.

Project structure

Until this release, the project was essentially a monorepo in disguise: the
core packages for handling matrices and computational graphs were accompanied
by many models implementations (from the very simple up to the most
sophisticated ones) and commands (models management utilities and servers).

We now prefer to keep in this very repository only the core components of spaGO,
only enriched with an (opinionated) set of popular models and functionalities.
Bigger sub-packages and related commands are moved to separate repositories.
The moved content includes, most notably, code related to Transformers
and Flair.
Please refer to the section Projects using Spago from the README for an
updated list of references to separate projects (note: some of them are still
work in progress).
If you have the feeling that something big is missing in spaGO, chances are
it was moved to one of these separate projects: just have a look there first.

The arrangement of packages has been simplified: there's no need anymore to
distinguish between cmd and pkg; all the main subpackages are located in
the project's root path. Similarly, many packages, previously nested under
pkg/ml, can now be found at root level too.

Go version and dependencies

The minimum required Go version is 1.18, primarily needed for the
introduction of type parameters (generics).

Thanks to the creation of separate projects, discussed above, and further
refactoring, the main set of required dependencies is limited to the ones
for testing.

Only the subpackage embeddings/store/diskstore requires something more, so
we defined it as "opt-in" submodule, with its own dependencies.

float32 vs. float64

Instead of separate packages mat32 and mat64, there is now a single unified
package mat. Many parts of the implementation make use of type parameters
(generics), however the package's public API makes a rather narrow use of them.

In particular, we abstained from adding type parameters to widely-used types,
such as the Matrix interface. Where suitable, we are simply favoring float64
values, the de-facto preferred floating point type in Go (just think about Go
math package). For other situations, we introduced a new subpackage
mat/float. It provides simple types, holding either float32 or float64
values, as scalars or slices, and makes it easy to convert values between
different precisions, all without making explicit use of generics.
This design prevents the excessive spreading of type arguments to tons of other
types that need to manipulate matrices, bot from other spaGO packages and
from your own code.

Matrices

• The type mat.Matrix is the primary interface for matrices and vectors
  throughout the project.
• The type mat.Dense is the concrete implementation for a dense matrix.
  Unlike the interface, it has a type argument to distinguish between float32
  and float64.
• We removed implementation and support for sparse matrices, since their
  efficacy and utility were marginal. A better implementation might come back
  in the future.
• A new dense matrix can be created "from scratch" by calling one of the several
  functions mat.New*** (NewDense, NewVecDense, ...). Here you must choose
  which data type to use, specifying it as type parameter (unless implicit).
• Once you have an existing matrix, you can create new instances preserving
  the same data type of the initial one: simply use one of the New*** methods
  on the matrix instance itself, rather than their top-level function
  counterparts.
• Any other operation performed on a matrix that creates a new instance will
  operate with the same type of the receiver, and returns an instance of that
  type too.
• Operations with matrices of different underlying data types are allowed, just
  beware the memory and computation overheads introduced by the necessary
  conversions.

Auto-grad package

• The package ag now implicitly works in "define-by-run" mode only.
  It's way more performant compared to the previous releases, and there would
  be no significant advantage in re-using a pre-defined graph ("define-and-run").
• There is no Graph anymore! At least, not as a first citizen: an implicit
  "virtual" graph is progressively formed each time an operation over some
  nodes is applied. The virtual graph can be observed by simply walking the
  tree of operations. Most methods of the former Graph are now simple
  functions in the ag package.
• We still provide a way to explicitly "free" some resources after use,
  both for helping the garbage collector and for returning some objects
  to their sync.Pool. The function ag.ReleaseGraph operates on the
  virtual graph described above, usually starting from the given output nodes.
• Forward operations are executed concurrently. As soon as an Operator is
  created (usually by calling one of the functions in ag, such as Add,
  Prod, etc.), the related Function's Forward procedure is performed
  on a new goroutine. Nevertheless, it's always safe to ask for the Operator's
  Value without worries: if it's called too soon, the function will lock
  until the result is computed, and only then return the value.
• To maximize performance, we removed the possibility to set a custom limit
  for concurrent computations. Thanks to the new design, we now let the Go
  runtime itself manage this problem for us, so that you can still limit
  and finetune concurrency with the GOMAXPROCS variable.
• The implementation of backpropagation is also redesigned and improved.
  Instead of invoking the backward procedure on an explicit Graph, you can call
  ag.Backward or ag.BackwardMany, specifying the output node (or nodes)
  of your computation (such as loss values, in traditional scenarios).
  The backward functions traverse the virtual graph and propagate the gradients,
  leveraging concurrency and making use of goroutines and locks in a way that's
  very similar to the forward procedure. The backward functions will lock and
  wait until the whole gradients propagation is complete before returning.
  The locking mechanism implemented in the nodes' Grad methods, will still
  prevent troubles in case your own code reads the gradients concurrently
  (that would be very uncommon).
• We also modified the implementation of time-steps handling and truncated
  backpropagation. Since we don't have the support of a concrete Graph
  structure anymore, we introduced a new dedicated type ag.TimeStepHandler,
  and related functions, such as NodeTimeStep. For performing a truncated
  backpropagation, we provide the function ag.BackwardT and
  ag.BackwardManyT: they work similarly to the normal backpropagation
  functions described above, only additionally requiring a time-step
  handler and the desired amount of back steps.
• We simplified and polished the API for creating new node-variables. Instead
  of having multiple functions for simple variables, scalars, constants,
  with/without name or grads, and various combination of those, you can now
  create any new variable with ag.Var, which accepts a Matrix value and
  creates a new node-variable with gradients accumulation disabled by default.
  To enable gradients propagation, or setting an explicit name (useful for
  model params or constants), you can use the Variable's chainable methods
  WithGrad and WithName. As a shortcut to create a scalar-matrix variable
  you can use ag.Scalar.
• The package ag/encoding provides generic structures and functions to obtain
  a sort of view of a virtual graph, with the goal of facilitating the
  encoding/marshaling of a graph in various formats.
  The package ag/encoding/dot is a rewriting of the former pkg/ml/graphviz,
  that uses the ag/encoding structures to represent a virtual graph in
  Graphviz DOT format.

Models

• As before, package nn provides types and functions for defining and
  handling models. Its subpackages are implementations of most common models.
  The set of built-in models has been remarkably revisited, moving some of them
  to separate projects, as previously explained.
• The Model interface has been extremely simplified: it only requires the
  special empty struct Module to be embedded in a model type. This is
  necessary only to distinguish an actual model from any other struct, which
  is especially useful for parameters traversal, or other similar operations.
• Since the Graph has been removed from ag, the models clearly don't need
  to hold a reference to it anymore. Similarly, there is no need for any other
  model-specific field, like the ones available from the former BaseModel.
  This implies the elimination of some seldomly used properties.
  Notable examples are the "processing mode" (from the old Graph) and the time
  step (from the old BaseModel).
  In situations where a removed value or feature is still needed, we suggest to
  either reintroduce the missing elements on the models that needs them, or
  to extract them to separate types and functions. An example of
  extracted behavior is the handling of time steps, already mentioned in the
  previous section.
• There is no distinction anymore between "pure" models and processors,
  making "reification" no longer necessary: once a model is created (or loaded),
  it can be immediately used, even for multiple concurrent inferences.
• A side effect of removing processor instances is that it's not possible
  to hold any sort of state related to a specific inference inside the
  structure of a model (or, at least, it's discouraged in most situations).
  Keeping track of a state is quite common for models that work with a running
  "memory" or cache. The recommended approach is to represent the state
  as a separate type, so that the "old" state can be passed as argument
  to the model's forward function (along with any other input), and the "new"
  or updated state can be returned from the same function (along with any other
  output).
  Some good examples can be observed in the implementation of recurrent
  networks (RNNs), located at nn/recurrent/...: each model has a single-step
  forward function (usually called Next) that accepts a previous state
  and returns a new one.
• We removed the Stack Model, in favor of a new simple function nn.Forward,
  that operates on a slice of StandardModel interfaces, connecting outputs to
  inputs sequentially for each module.
• We introduced the new type nn.Buffer: it's a Node implementation that does
  not require gradients, but can be serialized just like any other
  parameter. This is useful, for example, to store constants, to track the mean
  and std in batch norm layers, etc.
  As a shortcut to create a Buffer with a scalar-matrix value you can use
  nn.Const.
• We refactored the arguments of the parameters-traversal functions
  ForEachParam and ForEachParamStrict.
  Furthermore, the new interface ParamsTraverser allows to traverse a model's
  parameters that are not automatically discovered by the traversal functions
  via reflection. If a model implements this interface, the function
  TraverseParams will take precedence over the regular parameters visit.
• We introduced the function Apply, which visits all sub-models of any Model.
  Typical usages of this function include parameters initialization.

Embeddings

• The embeddings model has been refactored and made more flexible by
  splitting the new implementation into three main concerns: stores,
  the actual model, and the model's parameters.
• Raw embeddings data can be read from, and perhaps written to,
  virtually any suitable medium, be it in-memory, on-disk, local or remote
  services or databases, etc. The Store interface, defined in
  package embeddings/store, only requires an implementation to implement
  a bunch of read/write functions for key/value pairs. Both keys and values
  are just slice of bytes.
  For example, in a typical scenario involving word embeddings, a key might
  be a string word converted to []byte, and the value the byte-marshaled
  representation of a vector (or a more complex struct also holding
  other properties).
• It's not uncommon for a complex model, or application, to make use of
  more than one store. For a more convenient handling, multiple independent
  Stores can be organized together in a Repository, another interface
  defined in embeddings/store. A Repository is simply a provider for Stores,
  where each Store is identified by a string name.
  For example, if we are going to use a relational database for storing
  embeddings data, the Repository might establish the connection to the
  database, whereas each Store might identify a separate table by name,
  used for reading/writing data.
• We provide two built-in implementations of Repository/Store pairs.
  The package embeddings/store/diskstore is a Go submodule that stores data
  on disk, using BadgerDB; this is comparable to the implementation
  from previous releases.
  The package embeddings/store/memstore is a simple volatile in-memory
  implementation; among other usages, it might be especially convenient for
  testing.
• The package embeddings implements the main embeddings Model.
  One Model can read and write data to a single Store, obtained from a
  Repository by the configured name.
  The model delegates to the embeddings Store the responsibility to actually
  store the data; for this reason, the Store value on a Model is prevented
  from being serialized (this is done with the utility type
  embeddings/store.PreventStoreMarshaling).
• To facilitate different use cases, the Model allows a limited set of
  possible key types, using the constraint Key as type argument.
• The type Embedding represents a single embedding value that can be handled
  by a Model. It satisfies the interface nn.Param, allowing seamless
  integration with operations involving any other model. Behind the hood,
  the implementation takes care of reading/writing data against a
  Store, efficiently handling marshaling/unmarshaling and preventing
  race conditions. The Value and the Payload (if any) are read/written
  against the Store; the Grad is only kept in memory. All properties
  of different Embedding instances for the same key are kept
  synchronized upon changes.
• A Model keeps track of all Embedding parameters with associated gradients.
  The method TraverseParams allows these parameters to be discovered and
  seen as if they were any other regular type of parameter. This is
  especially important for operations such as embeddings optimization.
• It is a common practice to share the same embeddings among multiple models.
  In this case it is important that the serialized (and deserialized)
  instance is very same one. Therefore, we introduced the Shared structure
  that prevents binary marshaling.

Optimizers

• Gradient descent optimization algorithms are available under the package
  gd, with minor API changes.
• We removed other methods, such as differential evolution, planning to
  re-implement them on separate forthcoming projects.

Utilities

• We removed the formed package pkg/utils. Some of its content was related
  to functionalities now moved to separate projects. Any remaining useful code
  has been refactored and moved to more appropriate places.