aot autograd: dont allow symint outputs to get tangents in the bw graph (#96219)
Previously, if dynamic shapes were turned on and we had a forward graph that returns a symint, then we would generate a backward graph that takes in a tangent input for that symint fwd output. This causes problems for downstream - inductor will see an input that it expects to be a symint, but it gets a `None` from autograd.
Confirmed that this repro now passes:
```
benchmarks/dynamo/torchbench.py --devices cuda --inductor --dynamic-shapes --unspecialize-int --accuracy --training --only drq
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96219
Approved by: https://github.com/ezyang
