Adds error handling and reverts LmExample class to original

src/levanter/models/lm_model.py

@@ -26,6 +26,15 @@ class MaskedLmExample(eqx.Module):
     def masked_lm(
         tokens: hax.NamedArray, targets: hax.NamedArray, attn_mask: hax.NamedArray, ignore_id: Optional[int] = None
     ) -> "MaskedLmExample":
+        if tokens.ndim != 1:
+            raise ValueError("tokens must be a 1D array")
+
+        if not jnp.issubdtype(tokens.dtype, jnp.integer):
+            raise ValueError("tokens must be an integer array")
+
+        if tokens.shape != targets.shape:
+            raise ValueError("tokens and targets must have the same shape")
+
         Pos = tokens.axes[0]
 
         mask = tokens.array != targets.array
@@ -41,8 +50,7 @@ def masked_lm(
 class LmExample(eqx.Module):
     tokens: hax.NamedArray
     loss_mask: hax.NamedArray
-    attn_mask: hax.NamedArray
-    targets: Optional[hax.NamedArray] = None
+    attn_mask: AttentionMask | NamedArray = AttentionMask.causal()
 
     @staticmethod
     def causal(
@@ -66,27 +74,6 @@ def causal(
         attn_mask = AttentionMask.causal()
         return LmExample(tokens=tokens, loss_mask=loss_mask, attn_mask=attn_mask)
 
-    @staticmethod
-    def masked_lm(
-        tokens: hax.NamedArray, targets: hax.NamedArray, attn_mask: hax.NamedArray, ignore_id: Optional[int] = None
-    ) -> "LmExample":
-        Pos = tokens.axes[0]
-
-        mask = tokens.array != targets.array
-        loss_mask = mask.astype(jnp.float32)
-
-        if ignore_id is not None:
-            ignore_mask = targets.array != ignore_id
-            loss_mask = loss_mask * ignore_mask.astype(jnp.float32)
-
-        print(f"tokens shape: {tokens.shape}")
-        print(f"targets shape: {targets.shape}")
-        print(f"loss_mask shape: {loss_mask.shape}")
-        print(f"attn_mask shape: {attn_mask.shape}")
-
-        return LmExample(tokens=tokens, targets=targets, loss_mask=loss_mask, attn_mask=attn_mask)
-
-
 
 class LmConfig(draccus.PluginRegistry, abc.ABC, Generic[LmT], discover_packages_path="levanter.models"):  # type: ignore
     @property