Update torch::stable::Tensor() default constructor (#159507)

Allows things like

```cpp
Tensor cu_seqlens_q;
if (...) {
   cu_seqlens_q = ...
}
...
```

Also adds `torch::stable::Tensor.defined()`

Pull Request resolved: https://github.com/pytorch/pytorch/pull/159507
Approved by: https://github.com/janeyx99

test/cpp_extensions/libtorch_agnostic_extension/libtorch_agnostic/csrc/kernel.cpp

@@ -320,3 +320,38 @@ STABLE_TORCH_LIBRARY_FRAGMENT(libtorch_agnostic, m) {
 STABLE_TORCH_LIBRARY_IMPL(libtorch_agnostic, CPU, m) {
   m.impl("my_zero_", &boxed_my_zero_);
 }
+
+bool test_default_constructor(bool defined) {
+  Tensor out;
+  if (defined) {
+    AtenTensorHandle defined_ath;
+    int64_t sizes[] = {2, 3};
+    int64_t strides[] = {3, 1};
+    aoti_torch_empty_strided(
+        2,
+        sizes,
+        strides,
+        aoti_torch_dtype_float32(),
+        aoti_torch_device_type_cpu(),
+        0,
+        &defined_ath);
+    out = Tensor(defined_ath);
+  }
+  return out.defined();
+}
+
+void boxed_test_default_constructor(
+    StableIValue* stack,
+    uint64_t num_args,
+    uint64_t num_outputs) {
+  bool res = test_default_constructor(to<bool>(stack[0]));
+  stack[0] = from(res);
+}
+
+STABLE_TORCH_LIBRARY_FRAGMENT(libtorch_agnostic, m) {
+  m.def("test_default_constructor(bool undefined) -> bool");
+}
+
+STABLE_TORCH_LIBRARY_IMPL(libtorch_agnostic, CompositeExplicitAutograd, m) {
+  m.impl("test_default_constructor", &boxed_test_default_constructor);
+}

test/cpp_extensions/libtorch_agnostic_extension/libtorch_agnostic/ops.py

@@ -164,3 +164,15 @@ def fill_infinity(t) -> Tensor:
     Returns: The modified tensor (same as input)
     """
     return torch.ops.libtorch_agnostic.fill_infinity.default(t)
+
+
+def test_default_constructor(defined) -> bool:
+    """
+    Tests the default constructor for torch::stable::Tensor.
+
+    Args:
+        defined: bool - if True, tests defined tensor; if False, tests undefined tensor
+
+    Returns: bool - result of calling .defined() on the tensor
+    """
+    return torch.ops.libtorch_agnostic.test_default_constructor.default(defined)

test/cpp_extensions/libtorch_agnostic_extension/test/test_libtorch_agnostic.py

@@ -218,6 +218,20 @@ if not IS_WINDOWS:
             expected = torch.full_like(t, math.inf)
             self.assertEqual(out, expected)
 
+        @onlyCPU
+        def test_default_constructor(self):
+            import libtorch_agnostic
+
+            defined_tensor_is_defined = libtorch_agnostic.ops.test_default_constructor(
+                True
+            )
+            self.assertTrue(defined_tensor_is_defined)
+
+            undefined_tensor_is_defined = (
+                libtorch_agnostic.ops.test_default_constructor(False)
+            )
+            self.assertFalse(undefined_tensor_is_defined)
+
     instantiate_device_type_tests(TestLibtorchAgnostic, globals(), except_for=None)
 
 if __name__ == "__main__":