Slightly faster FX graph iterator (#121611)
Before:
```
iterating over 100000000 FX nodes took 5.9s (16830686 nodes/s)
```
After:
```
iterating over 100000000 FX nodes took 5.0s (19937698 nodes/s)
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121611
Approved by: https://github.com/oulgen
