Fix loading with only state dict and low_cpu_mem_usage = True (#35217)

* fix loading with only state dict and config

* style

* add tests

---------

Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

tests/utils/test_modeling_utils.py

@@ -1750,6 +1750,26 @@ class ModelUtilsTest(TestCasePlus):
                 new_model.generate(random_ids, max_new_tokens=3)
             self.assertTrue(len(w) == 0)
 
+    def test_load_model_with_state_dict_only(self):
+        model = BertModel.from_pretrained("hf-internal-testing/tiny-random-bert")
+        state_dict = model.state_dict()
+        config = model.config
+
+        model_loaded = BertModel.from_pretrained(
+            pretrained_model_name_or_path=None, config=config, state_dict=state_dict
+        )
+        self.assertTrue(check_models_equal(model, model_loaded))
+
+    def test_load_model_with_state_dict_only_low_cpu_mem_usage(self):
+        model = BertModel.from_pretrained("hf-internal-testing/tiny-random-bert")
+        state_dict = model.state_dict()
+        config = model.config
+
+        model_loaded = BertModel.from_pretrained(
+            pretrained_model_name_or_path=None, config=config, state_dict=state_dict, low_cpu_mem_usage=True
+        )
+        self.assertTrue(check_models_equal(model, model_loaded))
+
 
 @slow
 @require_torch