Fix Python-bound function signature (torch._C.Graph.addInput) (#88528)
In pytorch/torch/_C/__init__.pyi, Graph.addInput has signature
```python
def addInput(self, name: str) -> Value: ...
```
which doesn't match the corresponding function
```cpp
Value* addInput(const std::string& name = "") {
return block_->addInput(name);
}
```
in python_ir.cpp. This PR aligns the bound function on both C++ and Python sides. Without this PR, mypy will compain whenever a change contains some calls to `addInput`; for example,
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88528
Approved by: https://github.com/davidberard98
