Don't call item() into torch.scalar_tensor uselessly (#125373)

Fixes https://github.com/pytorch/pytorch/issues/125368

Signed-off-by: Edward Z. Yang <ezyang@meta.com>

Pull Request resolved: https://github.com/pytorch/pytorch/pull/125373
Approved by: https://github.com/Skylion007

test/dynamo/test_misc.py

@@ -4642,6 +4642,19 @@ def fn():
         res2 = opt_fn(x)
         self.assertEqual(res, res2)
 
+    @patch.object(torch._dynamo.config, "capture_scalar_outputs", True)
+    def test_tensor_ctor_list_of_tensor(self):
+        def fn(x):
+            return torch.tensor([x], dtype=torch.int64)
+
+        x = torch.tensor(20)
+        res = fn(x)
+        cnts = torch._dynamo.testing.CompileCounter()
+        opt_fn = torch._dynamo.optimize(cnts)(fn)
+        res2 = opt_fn(x)
+        self.assertEqual(res, res2)
+        self.assertEqual(cnts.frame_count, 1)
+
     def test_tensor_types(self):
         def fn(dtype, tensor_type):
             x = torch.empty(4, dtype=dtype)

torch/_refs/__init__.py

@@ -6359,9 +6359,8 @@ def _infer_scalar_type(obj):
 # xref: recursive_store in torch/csrc/utils/tensor_new.cpp
 def _recursive_build(scalarType: torch.dtype, obj: TensorOrNumberLikeType):
     if isinstance(obj, Tensor) and obj.ndim <= 1:
-        obj = obj.item()
-        # fall through into next case
-    if isinstance(obj, Number):
+        return obj.detach().to(dtype=scalarType, device="cpu", copy=True).view(())
+    elif isinstance(obj, Number):
         return torch.scalar_tensor(obj, dtype=scalarType)
 
     seq = obj