Unit tests loading from pretrained fill missing keys (#623)

* Add unit tests for fill_missing_keys

* Reformat test_loading_from_pretrained.py with black

* Rename unit test file to test_loading_from_pretrained_utilities to avoid naming conflict

tests/unit/test_loading_from_pretrained_utilities.py

@@ -0,0 +1,72 @@
+from unittest import mock
+
+import pytest
+
+from transformer_lens import HookedTransformer
+from transformer_lens.HookedTransformerConfig import HookedTransformerConfig
+from transformer_lens.loading_from_pretrained import fill_missing_keys
+
+
+def get_default_config():
+    return HookedTransformerConfig(
+        d_model=128, d_head=8, n_heads=16, n_ctx=128, n_layers=1, d_vocab=50257, attn_only=True
+    )
+
+
+# Successes
+
+
+@mock.patch("logging.warning")
+def test_fill_missing_keys(mock_warning):
+    cfg = get_default_config()
+    model = HookedTransformer(cfg)
+    default_state_dict = model.state_dict()
+
+    incomplete_state_dict = {k: v for k, v in default_state_dict.items() if "W_" not in k}
+
+    filled_state_dict = fill_missing_keys(model, incomplete_state_dict)
+
+    assert set(filled_state_dict.keys()) == set(default_state_dict.keys())
+
+    # Check that warnings were issued for missing weight matrices
+    for key in default_state_dict:
+        if "W_" in key and key not in incomplete_state_dict:
+            mock_warning.assert_any_call(
+                f"Missing key for a weight matrix in pretrained, filled in with an empty tensor: {key}"
+            )
+
+
+def test_fill_missing_keys_with_hf_model_keys():
+    cfg = get_default_config()
+    model = HookedTransformer(cfg)
+    default_state_dict = model.state_dict()
+
+    incomplete_state_dict = {k: v for k, v in default_state_dict.items() if "hf_model" not in k}
+
+    filled_state_dict = fill_missing_keys(model, incomplete_state_dict)
+
+    expected_keys = set(default_state_dict.keys()) - {
+        k for k in default_state_dict.keys() if "hf_model" in k
+    }
+    assert set(filled_state_dict.keys()) == expected_keys
+
+
+def test_fill_missing_keys_no_missing_keys():
+    cfg = get_default_config()
+    model = HookedTransformer(cfg)
+    default_state_dict = model.state_dict()
+
+    filled_state_dict = fill_missing_keys(model, default_state_dict)
+
+    assert filled_state_dict == default_state_dict
+
+
+# Failures
+
+
+def test_fill_missing_keys_raises_error_on_invalid_model():
+    invalid_model = None
+    default_state_dict = {}
+
+    with pytest.raises(AttributeError):
+        fill_missing_keys(invalid_model, default_state_dict)