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/eval_mlp.cpp

@@ -0,0 +1,25 @@
+// Copyright © 2024 Apple Inc.
+
+#include <mlx/mlx.h>
+#include <iostream>
+
+using namespace mlx::core;
+
+int main() {
+  int batch_size = 8;
+  int input_dim = 32;
+
+  // Make the input
+  random::seed(42);
+  auto example_x = random::uniform({batch_size, input_dim});
+
+  // Import the function
+  auto forward = import_function("eval_mlp.mlxfn");
+
+  // Call the imported function
+  auto out = forward({example_x})[0];
+
+  std::cout << out << std::endl;
+
+  return 0;
+}