Add simple quantization API (#586)

* Quantization draft

* Finish initial quantization API

* Docs

lib/axon.ex

@@ -855,6 +855,12 @@ defmodule Axon do
         use_bias: true
       ])
 
+    meta =
+      opts[:meta] ||
+        %{}
+        |> Map.put(:units, units)
+        |> Map.put(:use_bias, opts[:use_bias])
+
     kernel_shape = &Axon.Shape.dense_kernel(&1, units)
     bias_shape = &Axon.Shape.dense_bias(&1, units)
 
@@ -868,7 +874,7 @@ defmodule Axon do
         {[x, kernel], :dense}
       end
 
-    node = layer(op, inputs, name: opts[:name], meta: opts[:meta], op_name: :dense)
+    node = layer(op, inputs, name: opts[:name], meta: meta, op_name: :dense)
 
     if activation = opts[:activation] do
       activation(node, activation)
@@ -3666,7 +3672,7 @@ defmodule Axon do
   """
   @doc type: :graph
   def get_op_counts(%Axon{} = axon) do
-    reduce_nodes(axon, %{}, fn %Axon.Node{op: op}, op_counts ->
+    reduce_nodes(axon, %{}, fn %Axon.Node{op_name: op}, op_counts ->
       Map.update(op_counts, op, 1, fn x -> x + 1 end)
     end)
   end
@@ -4096,6 +4102,33 @@ defmodule Axon do
     end
   end
 
+  @doc """
+  Returns a mapping of layer names to layer properties.
+  """
+  def properties(%Axon{output: id, nodes: nodes}) do
+    {_, _, properties} = node_properties(id, nodes, {%{}, %{}, %{}})
+    properties
+  end
+
+  defp node_properties(id, nodes, {cache, op_counts, properties} = acc) do
+    case cache do
+      %{^id => _} ->
+        {cache, op_counts, properties}
+
+      %{} ->
+        %Axon.Node{parent: parents, name: name_fn, op_name: op_name} = nodes[id]
+
+        {cache, op_counts, properties} =
+          Enum.reduce(parents, acc, &node_properties(&1, nodes, &2))
+
+        name = name_fn.(op_name, op_counts)
+        op_counts = Map.update(op_counts, op_name, 1, fn x -> x + 1 end)
+        properties = Map.put(properties, name, op_name)
+
+        {Map.put(cache, id, name), op_counts, properties}
+    end
+  end
+
   ## Helpers
 
   @valid_initializers [:zeros, :ones, :uniform, :normal, :identity] ++

lib/axon/model_state.ex

@@ -206,6 +206,8 @@ defmodule Axon.ModelState do
 
   defp traverse(%Nx.Tensor{}, acc), do: [Enum.reverse(acc)]
 
+  defp traverse(%Axon.Quantization.QTensor{}, acc), do: [Enum.reverse(acc)]
+
   defp traverse(map, acc) do
     Enum.flat_map(map, fn {k, value} ->
       traverse(value, [k | acc])
@@ -273,6 +275,10 @@ defmodule Axon.ModelState do
           new_val = fun.(key, val_lhs, val_rhs)
           Map.put(acc, key, new_val)
 
+        %Axon.Quantization.QTensor{} = val_rhs ->
+          new_val = fun.(key, val_lhs, val_rhs)
+          Map.put(acc, key, new_val)
+
         val_rhs when is_map(val_lhs) and is_map(val_rhs) ->
           updated_val = tree_merge(val_lhs, val_rhs, fun)
           Map.put(acc, key, updated_val)
@@ -321,6 +327,11 @@ defmodule Axon.ModelState do
         {_, %Nx.Tensor{} = tensor}, {count, size} ->
           {count + Nx.size(tensor), size + Nx.byte_size(tensor)}
 
+        {_, %Axon.Quantization.QTensor{value: value, scale: scale, zero_point: zero}},
+        {count, size} ->
+          {count + Nx.size(value) + Nx.size(scale) + Nx.size(zero),
+           size + Nx.byte_size(value) + Nx.byte_size(scale) + Nx.byte_size(zero)}
+
         {_, map}, {count, size} ->
           {inner_count, inner_size} = get_param_info(map)
           {count + inner_count, size + inner_size}

lib/axon/quantization.ex

@@ -0,0 +1,161 @@
+defmodule Axon.Quantization do
+  @moduledoc """
+  Model quantization.
+
+  Model quantization is a technique for reducing the memory footprint of
+  a model by converting portions of a model to use quantized representations.
+  Typically, these quantized representations are low-precision integers.
+
+  This is an **experimental** API which implements weight-only quantization.
+  The implementation in this module will convert dense layers in a large
+  model to quantized-variants. The only supported quantization type is
+  `{:s, 8}`. Axon quantization is inference-only. Training is not currently
+  supported.
+  """
+  alias Axon.Quantization.Layers
+  alias Axon.Quantization.QTensor
+
+  @doc """
+  Quantizes a model and a model state.
+
+  Given a model and model state, this method will rewrite all
+  of the dense layers in the model to perform weight-only 8-bit
+  integer versions of the same operation. It will also replace values
+  for all dense kernels in the given model state with quantized
+  tensors.
+  """
+  def quantize(%Axon{} = model, %Axon.ModelState{} = model_state) do
+    quantized_model = quantize_model(model)
+    quantized_model_state = quantize_model_state(model, model_state)
+    {quantized_model, quantized_model_state}
+  end
+
+  @doc """
+  Replaces standard operations with quantized variants.
+
+  The only supported conversion is to convert regular dense layers
+  to a weight-only 8-bit integer variant. Note that this only replaces
+  the properties of the model. If you have a pre-trained model state
+  that you wish to quantize, refer to `Axon.Quantization.quantize_model_state/1`.
+
+  All `:dense` layers in the model are replaced with `Axon.Quantization.weight_only_quantized_dense/3`.
+  """
+  def quantize_model(%Axon{} = model) do
+    quantized_dense_rewriter = fn [%Axon{} = x], _output, units, use_bias ->
+      weight_only_quantized_dense(x, units, use_bias: use_bias)
+    end
+
+    Axon.rewrite_nodes(model, fn
+      %Axon.Node{op: :dense, meta: meta} ->
+        &quantized_dense_rewriter.(&1, &2, meta[:units], meta[:use_bias])
+
+      _ ->
+        :skip
+    end)
+  end
+
+  @doc """
+  Returns a quantized model state.
+
+  Given a model and a model state, this function will replace
+  all dense layer kernels with a quantized version of the weight.
+
+  Training is not currently supported, so all quantized layers are
+  automatically frozen.
+  """
+  def quantize_model_state(model, model_state) do
+    dense_layer_names =
+      model
+      |> Axon.properties()
+      |> Enum.filter(fn {_, v} -> v == :dense end)
+      |> Enum.map(fn {k, _} -> k end)
+      |> MapSet.new()
+
+    state =
+      Enum.reduce(dense_layer_names, model_state, fn layer_name, state ->
+        update_in(state, [Access.key!(:data), layer_name, "kernel"], &QTensor.from_tensor/1)
+      end)
+
+    Axon.ModelState.freeze(state, fn [name | _] ->
+      MapSet.member?(dense_layer_names, name)
+    end)
+  end
+
+  ## Layers
+
+  @doc """
+  Adds a weight-only quantized dense layer to the network.
+
+  This is equivalent to a dense layer, but works on quantized
+  weights for reducing model memory footprint.
+
+  Compiles to `Axon.Quantization.Layers.weight_only_quantized_dense/4`.
+
+  ## Options
+
+    * `:name` - layer name.
+
+    * `:kernel_initializer` - initializer for `kernel` weights.
+      Defaults to `:glorot_uniform`.
+
+    * `:bias_initializer` - initializer for `bias` weights. Defaults
+      to `:zeros`.
+
+    * `:use_bias` - whether the layer should add bias to the output.
+      Defaults to `true`.
+  """
+  def weight_only_quantized_dense(x, units, opts \\ []) do
+    opts =
+      Keyword.validate!(opts, [
+        :name,
+        :meta,
+        use_bias: true,
+        kernel_initializer: :glorot_uniform,
+        bias_initializer: :zeros
+      ])
+
+    meta =
+      opts[:meta] ||
+        %{}
+        |> Map.put(:units, units)
+        |> Map.put(:use_bias, opts[:use_bias])
+
+    kernel_shape = &Axon.Shape.dense_kernel(&1, units)
+    bias_shape = &Axon.Shape.dense_bias(&1, units)
+
+    kernel =
+      Axon.param("kernel", kernel_shape,
+        initializer: fn shape, type, key ->
+          fun =
+            case opts[:kernel_initializer] do
+              init when is_atom(init) ->
+                apply(Axon.Initializers, [])
+
+              fun when is_function(fun) ->
+                fun
+            end
+
+          tensor =
+            case fun do
+              fun when is_function(fun, 2) ->
+                fun.(shape, type)
+
+              fun when is_function(fun, 3) ->
+                fun.(shape, type, key)
+            end
+
+          QTensor.from_tensor(tensor)
+        end
+      )
+
+    {inputs, op} =
+      if opts[:use_bias] do
+        bias = Axon.param("bias", bias_shape, initializer: opts[:bias_initializer])
+        {[x, kernel, bias], &Layers.weight_only_quantized_dense/4}
+      else
+        {[x, kernel], &Layers.weight_only_quantized_dense/3}
+      end
+
+    Axon.layer(op, inputs, name: opts[:name], meta: meta, op_name: :dense)
+  end
+end

lib/axon/quantization/layers.ex

@@ -0,0 +1,43 @@
+defmodule Axon.Quantization.Layers do
+  @moduledoc """
+  Quantized Layer Implementations.
+  """
+  alias Axon.Quantization.QTensor
+  import Nx.Defn
+
+  @doc """
+  Weight-only quantized version of a dense layer.
+
+  It expects the input kernel to be an `Axon.Quantization.QTensor`.
+  """
+  deftransform weight_only_quantized_dense(input, kernel, bias \\ 0, opts \\ []) do
+    {bias, opts} =
+      case bias do
+        %Nx.Tensor{} = bias ->
+          {bias, opts}
+
+        bias when is_number(bias) ->
+          {bias, opts}
+
+        opts when is_list(opts) ->
+          {Nx.tensor(0), opts}
+
+        other ->
+          raise ArgumentError, "invalid bias, expected a tensor, got #{inspect(other)}"
+      end
+
+    weight_only_quantized_dense_impl(input, kernel, bias, opts)
+  end
+
+  defnp weight_only_quantized_dense_impl(
+          input,
+          %QTensor{value: kernel, scale: scale},
+          bias,
+          _opts
+        ) do
+    input
+    |> Nx.dot([Nx.rank(input) - 1], Nx.as_type(kernel, Nx.type(input)), [0])
+    |> Nx.multiply(scale)
+    |> Nx.add(bias)
+  end
+end