Export / import functions to / from a file (#1642)

* export and import functions

* refactor + works for few primitives

* nit

* allow primitives with state

* nit

* nit

* simplify serialize / deserialize

* fix for constants

* python bindings

* maybe fix serialize failure case

* add example

* more primitives, training kind of works

* same result for python and c++

* some fixes

* fix export

* template it up

* some simplificatoin

* rebase

* allow kwargs and multiple functions

* exporter

* more primitives for exporting

* deal with endianness

* handle invalid stream

* add docstring

examples/export/train_mlp.cpp

@@ -0,0 +1,35 @@
+// Copyright © 2024 Apple Inc.
+
+#include <mlx/mlx.h>
+#include <iostream>
+
+using namespace mlx::core;
+
+int main() {
+  int batch_size = 8;
+  int input_dim = 32;
+  int output_dim = 10;
+
+  auto state = import_function("init_mlp.mlxfn")({});
+
+  // Make the input
+  random::seed(42);
+  auto example_X = random::normal({batch_size, input_dim});
+  auto example_y = random::randint(0, output_dim, {batch_size});
+
+  // Import the function
+  auto step = import_function("train_mlp.mlxfn");
+
+  // Call the imported function
+  for (int it = 0; it < 100; ++it) {
+    state.insert(state.end(), {example_X, example_y});
+    state = step(state);
+    eval(state);
+    auto loss = state.back();
+    state.pop_back();
+    if (it % 10 == 0) {
+      std::cout << "Loss " << loss.item<float>() << std::endl;
+    }
+  }
+  return 0;
+}