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.py

@@ -0,0 +1,52 @@
+# Copyright © 2024 Apple Inc.
+
+import mlx.core as mx
+import mlx.nn as nn
+import mlx.utils
+
+
+class MLP(nn.Module):
+    """A simple MLP."""
+
+    def __init__(
+        self, num_layers: int, input_dim: int, hidden_dim: int, output_dim: int
+    ):
+        super().__init__()
+        layer_sizes = [input_dim] + [hidden_dim] * num_layers + [output_dim]
+        self.layers = [
+            nn.Linear(idim, odim)
+            for idim, odim in zip(layer_sizes[:-1], layer_sizes[1:])
+        ]
+
+    def __call__(self, x):
+        for l in self.layers[:-1]:
+            x = nn.relu(l(x))
+        return self.layers[-1](x)
+
+
+if __name__ == "__main__":
+
+    batch_size = 8
+    input_dim = 32
+    output_dim = 10
+
+    # Load the model
+    mx.random.seed(0)  # Seed for params
+    model = MLP(num_layers=5, input_dim=input_dim, hidden_dim=64, output_dim=output_dim)
+    mx.eval(model)
+
+    # Note, the model parameters are saved in the export function
+    def forward(x):
+        return model(x)
+
+    mx.random.seed(42)  # Seed for input
+    example_x = mx.random.uniform(shape=(batch_size, input_dim))
+
+    mx.export_function("eval_mlp.mlxfn", forward, example_x)
+
+    # Import in Python
+    imported_forward = mx.import_function("eval_mlp.mlxfn")
+    expected = forward(example_x)
+    (out,) = imported_forward(example_x)
+    assert mx.allclose(expected, out)
+    print(out)