Add parentheses to `FloorDiv`. (#98290)
[SymPy incorrectly prints](https://github.com/sympy/sympy/issues/25026) multiplications that:
- Have a negative term; and
- Have a custom operation of smaller precedence than `Mul` (except for `Add` -- which is expanded)
I have observed this behavior when running `maml` with dynamic shapes (errors out on `master`, though). There was, for example, the following guard:
```python
# vars[12].size()[2] == 2
# vars[0].size()[2] == 3
# x.size()[2] == 28
vars[12].size()[2] ** 2 - \
2 * vars[12].size()[2] * \
(-vars[0].size()[2] + (-vars[0].size()[2] + (x.size()[2] - vars[0].size()[2]) // 2 + 1) // 2 + 1) // 2
```
Which translates into:
```python
>>> 2 ** 2 - 2 * 2 * (-3 + (-3 + (28 - 3) // 2 + 1) // 2 + 1) // 2
>>> 2 ** 2 - 2 * 2 * (-3 + (-3 + 25 // 2 + 1) // 2 + 1) // 2
>>> 2 ** 2 - 2 * 2 * (-3 + 10 // 2 + 1) // 2
>>> 2 ** 2 - 2 * 2 * 3 // 2
>>> 4 - 2 * 2 * 3 // 2 # floordiv and mul have same precedence in Python
>>> -2
```
Now, placing the parentheses correctly (this PR), we get:
```python
>>> 4 - 2 * 2 * (3 // 2)
>>> 0
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98290
Approved by: https://github.com/ezyang
