Add the distributed linear layers

python/mlx/nn/layers/distributed.py

@@ -0,0 +1,156 @@
+# Copyright © 2024 Apple Inc.
+
+from functools import lru_cache
+from typing import Optional
+
+import mlx.core as mx
+from mlx.nn.layers.base import Module
+
+
+@lru_cache
+def sum_gradients(group):
+    if group.size() == 1:
+        return lambda x: x
+
+    @mx.custom_function
+    def f(x):
+        return x
+
+    @f.vjp
+    def f(x, dx, _):
+        return mx.distributed.all_sum(dx, group=group)
+
+    return f
+
+
+class AllToShardedLinear(Module):
+    """Each member of the group applies part of the affine transformation such
+    that the result is sharded across the group.
+
+    The gradients are automatically aggregated from each member of the group.
+
+    Args:
+        input_dims (int): The dimensionality of the input features
+        output_dims (int): The dimensionality of the output features
+        bias (bool, optional): If set to ``False`` the the layer will not use a
+            bias. Default is ``True``.
+        group (mx.distributed.Group, optional): The sharding will happen across
+            this group. If not set then the global group is used. Default is
+            ``None``.
+    """
+
+    def __init__(
+        self,
+        input_dims: int,
+        output_dims: int,
+        bias: bool = True,
+        group: Optional[mx.distributed.Group] = None,
+    ):
+        super().__init__()
+
+        # Initialize the parameters
+        scale = math.sqrt(1.0 / input_dims)
+        self.group = group or mx.distributed.init()
+        N = self.group.size()
+
+        if (output_dims % N) != 0:
+            raise ValueError(
+                f"Cannot shard the output of size {output_dims} across {N} devices."
+            )
+
+        self.weight = mx.random.uniform(
+            low=-scale,
+            high=scale,
+            shape=(output_dims // N, input_dims),
+        )
+        if bias:
+            self.bias = mx.random.uniform(
+                low=-scale,
+                high=scale,
+                shape=(output_dims // N,),
+            )
+
+    def _extra_repr(self) -> str:
+        N = self.group.size()
+        return f"input_dims={self.weight.shape[1]}, output_dims={N * self.weight.shape[0]}, bias={'bias' in self}"
+
+    def __call__(self, x: mx.array) -> mx.array:
+        # Aggregate the gradients coming from each shard
+        if self.group.size() > 1:
+            x = sum_gradients(self.group)(x)
+
+        # Compute the affine projection
+        if "bias" in self:
+            x = mx.addmm(self["bias"], x, self["weight"].T)
+        else:
+            x = x @ self["weight"].T
+        return x
+
+
+class ShardedToAllLinear(Module):
+    """Each member of the group applies part of the affine transformation and
+    then aggregates the results.
+
+    All nodes will have the same exact result after this layer.
+
+    Args:
+        input_dims (int): The dimensionality of the input features
+        output_dims (int): The dimensionality of the output features
+        bias (bool, optional): If set to ``False`` the the layer will not use a
+            bias. Default is ``True``.
+        group (mx.distributed.Group, optional): The sharding will happen across
+            this group. If not set then the global group is used. Default is
+            ``None``.
+    """
+
+    def __init__(
+        self,
+        input_dims: int,
+        output_dims: int,
+        bias: bool = True,
+        group: Optional[mx.distributed.Group] = None,
+    ):
+        super().__init__()
+
+        # Initialize the parameters
+        scale = math.sqrt(1.0 / input_dims)
+        self.group = group or mx.distributed.init()
+        N = self.group.size()
+
+        if (input_dims % N) != 0:
+            raise ValueError(
+                f"The input of size {input_dims} cannot be sharded across {N} devices."
+            )
+
+        self.weight = mx.random.uniform(
+            low=-scale,
+            high=scale,
+            shape=(output_dims, input_dims // N),
+        )
+        if bias:
+            self.bias = mx.random.uniform(
+                low=-scale,
+                high=scale,
+                shape=(output_dims,),
+            )
+
+    def _extra_repr(self) -> str:
+        N = self.group.size()
+        return f"input_dims={N * self.weight.shape[1]}, output_dims={self.weight.shape[0]}, bias={'bias' in self}"
+
+    def __call__(self, x: mx.array) -> mx.array:
+        if self.group.size() > 1:
+            # Perform the local projection and aggregate the results
+            x = x @ self["weight"].T
+            x = mx.distributed.all_sum(x, group=group)
+
+            # Add the bias if we have one
+            if "bias" in self:
+                x = x + self["bias"]
+        else:
+            # Normal linear layer as we are not in a distributed setting.
+            if "bias" in self:
+                x = mx.addmm(self["bias"], x, self["weight"].T)
+            else:
+                x = x @ self["weight"].T
+        return x