GPT2 Support (#798)

* GPT-2 model support

* Add test for gpt2 model

* Fix weight sanitizing for quantization

* use approx gelu

---------

Co-authored-by: Awni Hannun <awni@apple.com>

llms/tests/test_models.py

@@ -339,6 +339,22 @@ class TestModels(unittest.TestCase):
             model, args.model_type, args.vocab_size, args.num_hidden_layers
         )
 
+    def test_gpt2(self):
+        from mlx_lm.models import gpt2
+
+        args = gpt2.ModelArgs(
+            model_type="gpt2",
+            n_ctx=1024,
+            n_embd=768,
+            n_head=12,
+            n_layer=12,
+            n_positions=1024,
+            layer_norm_epsilon=1e-5,
+            vocab_size=50256,
+        )
+        model = gpt2.Model(args)
+        self.model_test_runner(model, args.model_type, args.vocab_size, args.n_layer)
+
     def test_openelm(self):
         from mlx_lm.models import openelm