Add HuggingFace arg so that arch is automatic

Stas Bekman had the idea of supporting a HuggingFace model as input so that all model architecture settings don't need manually dug up. We'd like something like:

```
python transformer_mem.py --hf_model_name_or_path meta-llama/Llama-2-7b-hf --num-gpus 8 --zero-stage 3 --batch-size-per-gpu 2 --sequence-length 4096
```