symbolic shapes: add parenthesis around FloorDiv expression (#91554)
Before it would print the guard expression like:
`2*3//2`
and now:
`2*(3//2)`
```python
print(2*3//2) # 3
print(2*(3//2)) # 2
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91554
Approved by: https://github.com/ezyang
