tree_map doesn't work over return_types

### 🐛 Describe the bug

```
import torch
from torch.utils._pytree import tree_map

print(tree_map(lambda a: None, torch.cummin(torch.randn(3), 0)))
```

This prints `None`.  I expected to get `torch.return_types.cummin(values=None, indices=None)`

cc @zou3519 

### Versions

master