Fix error message about enabling InferenceMode in Python (#105948)
Summary:
The old error message shows
```
... add `c10::InferenceMode mode;` before model.forward(). Note this guard is only available in C++ but not Python at present."
```
However InferenceMode for Python has been enabled since D28390595. It can be used as a context manager with `torch.inference_mode()`. The error message is fixed as so.
Test Plan: Easy
Reviewed By: yipjustin
Differential Revision: D47655392
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105948
Approved by: https://github.com/albanD
