fix(mlx-lm): olmo 1b model (#417)

llms/mlx_lm/models/olmo.py

@@ -24,6 +24,15 @@ class ModelArgs(BaseModelArgs):
     rope_theta: float = 10000
     rope_traditional: bool = False
     model_type: str = None
+    mlp_ratio: int = 4
+    weight_tying: bool = False
+
+    def __post_init__(self):
+        self.mlp_hidden_size = (
+            self.mlp_hidden_size
+            if self.mlp_hidden_size is not None
+            else self.mlp_ratio * self.d_model
+        )
 
 
 class LayerNorm(nn.LayerNorm):
@@ -99,6 +108,7 @@ class TransformerBlock(nn.Module):
         h = x + r
 
         x1, x2 = mx.split(self.ff_proj(self.ff_norm(h)), 2, axis=-1)
+
         out = h + self.ff_out(nn.silu(x2) * x1)
         return out, cache
 
@@ -107,9 +117,12 @@ class Transformer(nn.Module):
     def __init__(self, args: ModelArgs):
         super().__init__()
         self.n_layers = args.n_layers
+        self.weight_tying = args.weight_tying
+
         self.wte = nn.Embedding(args.embedding_size, args.d_model)
         self.blocks = [TransformerBlock(args=args) for _ in range(args.n_layers)]
-        self.ff_out = nn.Linear(args.d_model, args.embedding_size, bias=False)
+        if not self.weight_tying:
+            self.ff_out = nn.Linear(args.d_model, args.embedding_size, bias=False)
         self.norm = LayerNorm(args.d_model, affine=False)
 
     def __call__(
@@ -130,7 +143,12 @@ class Transformer(nn.Module):
         for e, block in enumerate(self.blocks):
             h, cache[e] = block(h, mask, cache[e])
 
-        return self.ff_out(self.norm(h)), cache
+        h = self.norm(h)
+
+        if self.weight_tying:
+            return h @ self.wte.weight.T, cache
+
+        return self.ff_out(h), cache
 
 
 class OlmoModel(nn.Module):