export docs (#1747)

docs/src/index.rst

@@ -45,6 +45,7 @@ are the CPU and GPU.
    usage/numpy
    usage/distributed
    usage/using_streams
+   usage/export
 
 .. toctree::
    :caption: Examples

docs/src/usage/compile.rst

@@ -422,6 +422,10 @@ the most opportunity to optimize the computation graph:
   # be faster even though the inner functions are compiled
   fun = mx.compile(outer)
 
+
+
+.. _shapeless_compile:
+
 Shapeless Compilation
 ---------------------
 

docs/src/usage/export.rst

@@ -0,0 +1,288 @@
+.. _export_usage:
+
+Exporting Functions
+===================
+
+.. currentmodule:: mlx.core
+
+MLX has an API to export and import functions to and from a file. This lets you
+run computations written in one MLX front-end (e.g. Python) in another MLX
+front-end (e.g. C++). 
+
+This guide walks through the basics of the MLX export API with some examples.
+To see the full list of functions check-out the :ref:`API documentation
+<export>`.
+
+Basics of Exporting 
+-------------------
+
+Let's start with a simple example:
+ 
+.. code-block:: python
+
+  def fun(x, y):
+    return x + y
+
+  x = mx.array(1.0)
+  y = mx.array(1.0)
+  mx.export_function("add.mlxfn", fun, x, y)
+
+To export a function, provide sample input arrays that the function
+can be called with. The data doesn't matter, but the shapes and types of the
+arrays do. In the above example we exported ``fun`` with two ``float32``
+scalar arrays. We can then import the function and run it:
+
+.. code-block:: python
+
+  add_fun = mx.import_function("add.mlxfn")
+
+  out, = add_fun(mx.array(1.0), mx.array(2.0))
+  # Prints: array(3, dtype=float32)
+  print(out)
+
+  out, = add_fun(mx.array(1.0), mx.array(3.0))
+  # Prints: array(4, dtype=float32)
+  print(out)
+
+  # Raises an exception
+  add_fun(mx.array(1), mx.array(3.0))
+
+  # Raises an exception
+  add_fun(mx.array([1.0, 2.0]), mx.array(3.0))
+
+Notice the third and fourth calls to ``add_fun`` raise exceptions because the
+shapes and types of the inputs are different than the shapes and types of the
+example inputs we exported the function with.
+
+Also notice that even though the original ``fun`` returns a single output
+array, the imported function always returns a tuple of one or more arrays.
+
+The inputs to :func:`export_function` and to an imported function can be
+specified as variable positional arguments or as a tuple of arrays:
+
+.. code-block:: python
+
+  def fun(x, y):
+    return x + y
+
+  x = mx.array(1.0)
+  y = mx.array(1.0)
+   
+  # Both arguments to fun are positional
+  mx.export_function("add.mlxfn", fun, x, y)
+
+  # Same as above
+  mx.export_function("add.mlxfn", fun, (x, y))
+
+  imported_fun = mx.import_function("add.mlxfn")
+
+  # Ok
+  out, = imported_fun(x, y)
+
+  # Also ok
+  out, = imported_fun((x, y))
+
+You can pass example inputs to functions as positional or keyword arguments. If
+you use keyword arguments to export the function, then you have to use the same
+keyword arguments when calling the imported function.
+
+.. code-block:: python
+
+  def fun(x, y):
+    return x + y
+
+  # One argument to fun is positional, the other is a kwarg
+  mx.export_function("add.mlxfn", fun, x, y=y)
+
+  imported_fun = mx.import_function("add.mlxfn")
+
+  # Ok
+  out, = imported_fun(x, y=y)
+
+  # Also ok
+  out, = imported_fun((x,), {"y": y})
+
+  # Raises since the keyword argument is missing
+  out, = imported_fun(x, y)
+
+  # Raises since the keyword argument has the wrong key
+  out, = imported_fun(x, z=y)
+
+
+Exporting Modules
+-----------------
+
+An :obj:`mlx.nn.Module` can be exported with or without the parameters included
+in the exported function. Here's an example:
+
+.. code-block:: python
+
+   model = nn.Linear(4, 4)
+   mx.eval(model.parameters())
+
+   def call(x):
+      return model(x)
+
+   mx.export_function("model.mlxfn", call, mx.zeros(4))
+
+In the above example, the :obj:`mlx.nn.Linear` module is exported. Its
+parameters are also saved to the ``model.mlxfn`` file.
+
+.. note::
+
+   For enclosed arrays inside an exported function, be extra careful to ensure
+   they are evaluated. The computation graph that gets exported will include
+   the computation that produces enclosed inputs.
+  
+   If the above example was missing ``mx.eval(model.parameters()``, the
+   exported function would include the random initialization of the
+   :obj:`mlx.nn.Module` parameters.
+
+If you only want to export the ``Module.__call__`` function without the
+parameters, pass them as inputs to the ``call`` wrapper:
+
+.. code-block:: python
+
+   model = nn.Linear(4, 4)
+   mx.eval(model.parameters())
+
+   def call(x, **params):
+     # Set the model's parameters to the input parameters
+     model.update(tree_unflatten(list(params.items())))
+     return model(x)
+ 
+   params = dict(tree_flatten(model.parameters()))
+   mx.export_function("model.mlxfn", call, (mx.zeros(4),), params)
+
+
+Shapeless Exports
+-----------------
+
+Just like :func:`compile`, functions can also be exported for dynamically shaped
+inputs. Pass ``shapeless=True`` to :func:`export_function` or :func:`exporter`
+to export a function which can be used for inputs with variable shapes:
+
+.. code-block:: python
+
+  mx.export_function("fun.mlxfn", mx.abs, mx.array(0.0), shapeless=True)
+  imported_abs = mx.import_function("fun.mlxfn")
+
+  # Ok
+  out, = imported_abs(mx.array(-1.0))
+  
+  # Also ok 
+  out, = imported_abs(mx.array([-1.0, -2.0]))
+
+With ``shapeless=False`` (which is the default), the second call to
+``imported_abs`` would raise an exception with a shape mismatch.
+
+Shapeless exporting works the same as shapeless compilation and should be
+used carefully. See the :ref:`documentation on shapeless compilation
+<shapeless_compile>` for more information.
+
+Exporting Multiple Traces
+-------------------------
+
+In some cases, functions build different computation graphs for different
+input arguments. A simple way to manage this is to export to a new file with
+each set of inputs. This is a fine option in many cases. But it can be
+suboptimal if the exported functions have a large amount of duplicate constant
+data (for example the parameters of a :obj:`mlx.nn.Module`).
+
+The export API in MLX lets you export multiple traces of the same function to
+a single file by creating an exporting context manager with :func:`exporter`:
+
+.. code-block:: python
+
+  def fun(x, y=None):
+      constant = mx.array(3.0)
+      if y is not None:
+        x += y 
+      return x + constant
+
+  with mx.exporter("fun.mlxfn", fun) as exporter:
+      exporter(mx.array(1.0))
+      exporter(mx.array(1.0), y=mx.array(0.0))
+
+  imported_function = mx.import_function("fun.mlxfn")
+
+  # Call the function with y=None
+  out, = imported_function(mx.array(1.0))
+  print(out)
+
+  # Call the function with y specified
+  out, = imported_function(mx.array(1.0), y=mx.array(1.0))
+  print(out)
+
+In the above example the function constant data, (i.e. ``constant``), is only
+saved once. 
+
+Transformations with Imported Functions
+---------------------------------------
+
+Function transformations like :func:`grad`, :func:`vmap`, and :func:`compile` work
+on imported functions just like regular Python functions:
+
+.. code-block:: python
+
+  def fun(x):
+      return mx.sin(x)
+
+  x = mx.array(0.0)
+  mx.export_function("sine.mlxfn", fun, x)
+
+  imported_fun = mx.import_function("sine.mlxfn")
+
+  # Take the derivative of the imported function
+  dfdx = mx.grad(lambda x: imported_fun(x)[0])
+  # Prints: array(1, dtype=float32)
+  print(dfdx(x))
+
+  # Compile the imported function 
+  mx.compile(imported_fun)
+  # Prints: array(0, dtype=float32)
+  print(compiled_fun(x)[0])
+
+
+Importing Functions in C++
+--------------------------
+
+Importing and running functions in C++ is basically the same as importing and
+running them in Python. First, follow the :ref:`instructions <mlx_in_cpp>` to
+setup a simple C++ project that uses MLX as a library.
+
+Next, export a simple function from Python:
+
+.. code-block:: python
+
+  def fun(x, y):
+      return mx.exp(x + y)
+
+  x = mx.array(1.0)
+  y = mx.array(1.0)
+  mx.export_function("fun.mlxfn", fun, x, y)
+
+
+Import and run the function in C++ with only a few lines of code:
+
+.. code-block:: c++
+
+  auto fun = mx::import_function("fun.mlxfn");
+
+  auto inputs = {mx::array(1.0), mx::array(1.0)};
+  auto outputs = fun(inputs);
+
+  // Prints: array(2, dtype=float32)
+  std::cout << outputs[0] << std::endl;
+
+Imported functions can be transformed in C++ just like in Python. Use 
+``std::vector<mx::array>`` for positional arguments and ``std::map<std::string,
+mx::array>`` for keyword arguments when calling imported functions in C++.
+
+More Examples
+-------------
+
+Here are a few more complete examples exporting more complex functions from
+Python and importing and running them in C++:
+
+* `Inference and training a multi-layer perceptron <https://github.com/ml-explore/mlx/tree/main/examples/export>`_

mlx/export.cpp

@@ -729,8 +729,8 @@ std::vector<array> ImportedFunction::operator()(
     msg << "[import_function::call] No imported function found which matches "
         << "the given positional and keyword arguments. Possible functions include:\n";
     ftable->print_functions(msg);
-    msg << "\nReceived function with " << args.size()
-        << " positional inputs and " << kwargs.size() << " keyword inputs:\n";
+    msg << "\nCalled with " << args.size() << " positional inputs and "
+        << kwargs.size() << " keyword inputs:\n";
     for (int i = 0; i < args.size(); ++i) {
       auto& in = args[i];
       msg << "  " << i + 1 << ": " << in.shape() << " " << in.dtype() << "\n";