Added support for sharing layers between different parts of a model.

docs/api/nn/shared.md

@@ -0,0 +1,7 @@
+# Sharing layers
+
+::: equinox.nn.Shared
+    selection:
+        members:
+            - __init__
+            - __call__

equinox/nn/__init__.py

@@ -39,6 +39,7 @@
     Sequential as Sequential,
     StatefulLayer as StatefulLayer,
 )
+from ._shared import Shared as Shared
 from ._spectral_norm import SpectralNorm as SpectralNorm
 from ._stateful import (
     delete_init_state as delete_init_state,

equinox/nn/_shared.py

@@ -0,0 +1,158 @@
+from collections.abc import Callable
+
+from jaxtyping import PyTree
+
+from .._eval_shape import filter_eval_shape
+from .._module import Module
+from .._tree import tree_at, tree_equal
+
+
+class SharedNode:
+    """Placeholder value for nodes that have been removed by `eqx.nn.Shared`."""
+
+    def __repr__(self):
+        return "SharedNode"
+
+
+class Shared(Module):
+    """Used to tie together multiple nodes across a PyTree.
+
+    Note that Equinox modules are Py**Trees** -- so the same layer, appearing in two
+    difference parts of the tree, will be treated as two copies of this layer. For
+    example,
+    ```python
+    class SubModel(eqx.Module):
+        linear: eqx.nn.Linear
+
+    class Model(eqx.Module):
+        linear: eqx.nn.Linear
+        submodel: SubModel
+
+        def __init__(self):
+            linear = eqx.nn.Linear(...)
+            self.linear = linear
+            self.submodel = SubModel(linear)
+    ```
+    is used to declare `model.linear` and `model.submodel.linear` as two separate
+    layers. They will start with the same initial parameter values, and then update
+    independently during training.
+
+    For when we really do want to share layers or weights across different parts of a
+    model, then `eqx.nn.Shared` exists as a way to easily express this in the PyTree
+    paradigm.
+
+    !!! Example
+
+        It is common in many language models to have an initial embedding matrix at the
+        start, and then to reuse this as the weight of the final linear transformation.
+
+        ```python
+        import equinox as eqx
+        import jax.numpy as jnp
+        from jaxtyping import Array, Int
+
+        class LanguageModel(eqx.Module):
+            shared: eqx.nn.Shared
+
+            def __init__(self):
+                embedding = eqx.nn.Embedding(...)
+                linear = eqx.nn.Linear(...)
+                # These two weights will now be tied together.
+                where = lambda embed_and_lin: embed_and_lin[1].weight
+                get = lambda embed_and_lin: embed_and_lin[0].weight
+                self.shared = eqx.nn.Shared((embedding, linear), where, get)
+
+            def __call__(self, tokens: Int[Array, "sequence"]):
+                # Expand back out so we can evaluate these layers.
+                embedding, linear = self.shared()
+                assert embedding.weight is linear.weight  # same parameter!
+                # Now go ahead and evaluate your language model.
+                values = jax.vmap(embedding)(tokens)
+                ...  # other layers, probably
+                return jax.vmap(linear)(values)
+        ```
+
+        _(Side note: you will sometimes see some authors referring to transposing
+        the embedding matrix prior to the final linear layer. This is because some
+        other libraries store the weight matrices of linear layers the other way
+        around. If that had been necessary here then we could have done it with
+        `get = lambda embed_and_lin: jnp.transpose(embed_and_lin[0].weight)`.)_
+
+    """
+
+    pytree: PyTree
+    where: Callable
+    get: Callable
+
+    def __init__(self, pytree: PyTree, where: Callable, get: Callable):
+        """**Arguments:**
+
+        - `pytree`: The PyTree to share some nodes across.
+        - `where`: a function specifying either a single node, or a sequence of nodes,
+            as with `eqx.tree_at(where, pytree, ...)`.
+        - `get`: a function, which when evaluated on `pytree`, returns either a single
+            value (if `where` does), or a sequence of values (if `where` does, and in
+            this case this must be a sequence of the same length as `where`).
+
+        The node(s) of `get(pytree)` and the corresponding value(s) of `where(pytree)`
+        will be tied together.
+
+        !!! info
+
+            To explain how this works. The implementation is just:
+            ```python
+            class Shared(eqx.Module):
+                pytree: PyTree
+                where: Callable
+                get: Callable
+
+                def __init__(self, pytree, where, get):
+                    # `0` is just some dummy value
+                    self.pytree = eqx.tree_at(where, pytree, replace_fn=lambda _: 0)
+                    self.where = where
+                    self.get = get
+
+                def __call__(self):
+                    return eqx.tree_at(self.where, self.pytree, self.get(self.pytree))
+            ```
+            so that at `__init__` time, the duplicate nodes specified in `where` are
+            removed from the PyTree. We no longer have a separate copy updating during
+            training.
+
+            And then at `__call__` time, references to the values returned by
+            `get(pytree)` are put in their place. We end up with a pytree of the same
+            structure as what we started with, which we can now use (evaluate as a
+            layer etc.) as normal.
+
+        !!! tip
+
+            If you need to apply any transform (e.g. transposing a matrix), then this
+            can be done as part of `get`. For example,
+            `get = lambda pair: jnp.transpose(pair[1].weight)`.
+        """
+
+        source_struct = filter_eval_shape(get, pytree)
+        dest_struct = filter_eval_shape(where, pytree)
+        if tree_equal(source_struct, dest_struct) is not True:
+            raise ValueError(
+                "Every node being shared together must have the same pytree "
+                "structure, shape+dtype of arrays, etc., as each other. Got:\n"
+                f"{source_struct}\n"
+                "and\n"
+                f"{dest_struct}"
+            )
+        self.pytree = tree_at(where, pytree, replace_fn=lambda _: SharedNode())
+        self.where = where
+        self.get = get
+
+    def __call__(self):
+        """**Arguments:**
+
+        None.
+
+        **Returns:**
+
+        A PyTree of the same structure as the original `pytree`, with `get(pytree)` in
+        the place of the nodes at `where(pytree)`.
+        """
+        return tree_at(self.where, self.pytree, self.get(self.pytree))

mkdocs.yml

@@ -128,6 +128,7 @@ nav:
             - 'api/nn/mlp.md'
             - 'api/nn/sequential.md'
             - 'api/nn/inference.md'
+            - 'api/nn/shared.md'
             - 'api/nn/stateful.md'
         - Filtering:
             - 'api/filtering/partition-combine.md'

tests/test_shared.py

@@ -0,0 +1,125 @@
+import jax
+import jax.numpy as jnp
+import jax.random as jr
+import pytest
+from jaxtyping import Array, Float, Int
+
+import equinox as eqx
+
+
+def test_shared_array(getkey):
+    class MyModule(eqx.Module):
+        shared: eqx.nn.Shared
+
+        def __init__(self):
+            embedding = eqx.nn.Embedding(
+                num_embeddings=3, embedding_size=4, key=getkey()
+            )
+            head = eqx.nn.Linear(4, 3, key=getkey())
+            where = lambda pair: pair[1].weight
+            get = lambda pair: pair[0].weight
+            self.shared = eqx.nn.Shared((embedding, head), where, get)
+
+        def __call__(self, token: Int[Array, ""]):
+            nonlocal called
+            called = True
+            embedding, head = self.shared()
+            assert embedding.weight is head.weight
+            return head(embedding(token))
+
+    called = False
+    module = MyModule()
+    module(jnp.array(0))
+    assert called
+
+
+# We share a non-leaf node
+def test_shared_node(getkey):
+    class MyModule(eqx.Module):
+        shared: eqx.nn.Shared
+
+        def __init__(self):
+            attention = eqx.nn.MultiheadAttention(
+                num_heads=3, query_size=12, key=getkey()
+            )
+            my_proj = eqx.nn.Linear(12, 12, use_bias=False, key=getkey())
+            where = lambda pair: pair[1].key_proj
+            get = lambda pair: pair[0]
+            self.shared = eqx.nn.Shared((my_proj, attention), where, get)
+
+        def __call__(self, x: Float[Array, "seq 12"]):
+            nonlocal called
+            called = True
+            my_proj, attention = self.shared()
+            eq = eqx.tree_equal(my_proj, attention.key_proj)
+            x = attention(x, x, x)
+            out = jax.vmap(my_proj)(x)
+            return out, eq
+
+    called = False
+    module = MyModule()
+    x = jr.normal(getkey(), (5, 12))
+
+    @eqx.filter_jit
+    @eqx.filter_grad(has_aux=True)
+    def f(module, x):
+        out, eq = module(x)
+        return jnp.sum(out), eq
+
+    d_module, eq = f(module, x)
+    assert called
+    assert eq
+    module = eqx.apply_updates(module, d_module)
+    d_module, eq = f(module, x)
+    assert eq
+    module = eqx.apply_updates(module, d_module)
+
+
+def test_mismatched_structure(getkey):
+    x = jr.normal(getkey(), (3, 4))
+    y = jr.normal(getkey(), (4, 3))
+    with pytest.raises(ValueError, match="Every node being shared together"):
+        eqx.nn.Shared((x, y), lambda pair: pair[0], lambda pair: pair[1])
+
+
+def test_multi_shared(getkey):
+    class MyModule(eqx.Module):
+        shared: eqx.nn.Shared
+
+        def __init__(self):
+            my_proj = eqx.nn.Linear(12, 12, use_bias=False, key=getkey())
+            attention = eqx.nn.MultiheadAttention(
+                num_heads=3, query_size=12, key=getkey()
+            )
+            where = lambda pair: (pair[1].key_proj, pair[1].query_proj.weight)
+            get = lambda pair: (pair[0], pair[0].weight + 1)
+            self.shared = eqx.nn.Shared((my_proj, attention), where, get)
+
+        def __call__(self, x: Float[Array, "seq 12"]):
+            nonlocal called
+            called = True
+            my_proj, attention = self.shared()
+            eq1 = eqx.tree_equal(my_proj, attention.key_proj)
+            eq2 = (my_proj.weight + 1 == attention.query_proj.weight).all()
+            x = attention(x, x, x)
+            out = jax.vmap(my_proj)(x)
+            eq = eq1 & eq2
+            return out, eq
+
+    called = False
+    module = MyModule()
+    x = jr.normal(getkey(), (5, 12))
+
+    @eqx.filter_jit
+    @eqx.filter_grad(has_aux=True)
+    def f(module, x):
+        out, eq = module(x)
+        return jnp.sum(out), eq
+
+    d_module, eq = f(module, x)
+    assert called
+    assert eq
+    module = eqx.apply_updates(module, d_module)
+    d_module, eq = f(module, x)
+    assert eq
+    module = eqx.apply_updates(module, d_module)