Add type hint for retains_grad (pytorch#103528)

Fixes pytorch#103485

Type checkers don't know about the existence of `retains_grad` otherwise:

```python
torch.randn(10, 10).retains_grad  # Cannot access member "retains_grad" for type "Tensor"
```

Pull Request resolved: pytorch#103528
Approved by: https://github.com/albanD, https://github.com/Skylion007, https://github.com/janeyx99

torch/_C/__init__.pyi.in

@@ -1455,6 +1455,7 @@ class _TensorMeta(type): ...
 # Defined in torch/csrc/autograd/python_variable.cpp
 class _TensorBase(metaclass=_TensorMeta):
     requires_grad: _bool
+    retains_grad: _bool
     shape: Size
     data: Tensor
     names: List[str]