[dynamo] Graph break on pack_padded_sequence (#108096)
This is to workaround #93501.
Fixes errors in:
```
./benchmarks/dynamo/torchbench.py --inference --performance --no-skip --inductor --freezing --only tacotron2
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108096
Approved by: https://github.com/davidberard98
