[WIP - Benchmark] Add generate function
This PR adds the `generate` function to Benchmarks. You can try it out by running:

```
python examples/benchmarking/run_benchmark.py --models gpt2 --batch_sizes 1 --sequence_lengths 20 --no_inference --generate
```

## TODO
- [ ] Add for TF as well
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
