[dynamo] Fix dict.get with no default (#115048)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/115048
Approved by: https://github.com/eellison, https://github.com/oulgen
ghstack dependencies: #114830, #115047

benchmarks/dynamo/ci_expected_accuracy/aot_eager_torchbench_inference.csv

@@ -118,7 +118,7 @@ hf_T5,pass,0
 
 
 
-hf_T5_generate,pass,19
+hf_T5_generate,pass,18
 
 
 

benchmarks/dynamo/ci_expected_accuracy/dynamo_eager_torchbench_inference.csv

@@ -118,7 +118,7 @@ hf_T5,pass,0
 
 
 
-hf_T5_generate,pass,19
+hf_T5_generate,pass,18
 
 
 

benchmarks/dynamo/ci_expected_accuracy/inductor_torchbench_inference.csv

@@ -118,7 +118,7 @@ hf_T5,pass,0
 
 
 
-hf_T5_generate,pass,19
+hf_T5_generate,pass,18
 
 
 

test/dynamo/test_functions.py

@@ -689,6 +689,7 @@ class FunctionTests(torch._dynamo.test_case.TestCase):
     @make_test
     def test_dict_ops(a, b):
         tmp = {"a": a + 1, "b": b + 2}
+        assert tmp.get("zzz") is None
         v = tmp.pop("b") + tmp.get("a") + tmp.get("missing", 3) + tmp.pop("missing", 4)
         tmp.update({"d": 3})
         tmp["c"] = v + tmp["d"]

torch/_dynamo/variables/dicts.py

@@ -147,11 +147,20 @@ class ConstDictVariable(VariableTracker):
         elif (
             name in ("pop", "get")
             and len(args) == 2
+            and not kwargs
             and ConstDictVariable.is_valid_key(args[0])
             and ConstDictVariable.get_key(args[0]) not in self.items
         ):
             # missing item, return the default value
             return args[1]
+        elif (
+            name == "get"
+            and len(args) == 1
+            and not kwargs
+            and ConstDictVariable.is_valid_key(args[0])
+            and ConstDictVariable.get_key(args[0]) not in self.items
+        ):
+            return ConstantVariable(None)
         elif (
             name == "pop"
             and args