Add tests for GPT2

tests/test_models.py

+import lm_eval.models as models
+import lm_eval.base as base
+
+def test_gpt2():
+    gpt2 = models.get_model('gpt2')(device="cpu")
+    (ll_dog, ig_dog), (ll_cat, ig_cat) = gpt2.loglikelihood([
+        ('The quick brown fox jumps over the lazy', ' dog'),
+        ('The quick brown fox jumps over the lazy', ' cat'),
+    ])
+
+    assert ll_dog > ll_cat
+    assert not ig_cat
+