Add Flux benchmark
This adds a benchmark for the Flux image generation pipeline.
Specifically, it only benchmarks the diffusion transformer (and omits
the text encoder and vae, which don't take up much time for the e2e
generation in Flux).
Needs https://github.com/pytorch/pytorch/pull/168176 to run in pytorch
repo:
```
python ./benchmarks/dynamo/torchbench.py --accuracy --inference --backend=inductor --only flux
python ./benchmarks/dynamo/torchbench.py --performance --inference --backend=inductor --only flux
```
