enable correct padding_idx for embedding layers (#1527)

llmfoundry/models/mpt/modeling_mpt.py

@@ -396,6 +396,7 @@ def __init__(self, config: MPTConfig):
         self.wte = SharedEmbedding(
             config.vocab_size,
             config.d_model,
+            padding_idx=config.pad_token_id,
             device=config.init_device,
         )
         if self.learned_pos_emb:

llmfoundry/models/utils/param_init_fns.py

@@ -224,6 +224,9 @@ def embedding_init(
             emb_init_fn_ = init_fn_
 
         emb_init_fn_(module.weight)
+        if module.padding_idx is not None:
+            with torch.no_grad():
+                module.weight[module.padding_idx].fill_(0)
 
         return True
 

tests/models/utils/test_param_init_fns.py

@@ -199,3 +199,30 @@ def test_emb_init(emb_init_cfg: Optional[tuple[str, Union[int, list[int]]]]):
                 emb_init_uniform_lim,
             ) == 2 and emb_init_uniform_lim[0] == emb_init_uniform_lim[1]:
                 assert (model.emb.weight == emb_init_uniform_lim[0]).all()
+
+
+@pytest.mark.parametrize(
+    'padding_idx',
+    [0, 2],
+)
+def test_emb_padding_init(padding_idx: int,):
+    cfg: dict[str, Union[int, list[int]]] = {
+        'vocab_size': 64,
+        'in_features': 16,
+        'n_layers': 2,
+        'padding_idx': padding_idx,
+        'emb_init_std': 5,
+    }
+    dict_cfg = om.create(cfg)
+
+    model = nn.Embedding(
+        dict_cfg.vocab_size,
+        dict_cfg.in_features,
+        dict_cfg.padding_idx,
+    )
+
+    model.apply(partial(param_init_fns.get('kaiming_normal_'), **dict_cfg))
+    assert isinstance(model, torch.nn.Embedding)
+
+    if dict_cfg.get('emb_init_std') is not None:
+        assert (model.weight[padding_idx] == 0).all()