Clean up hydra configs: rename 7b to 8b, remove experiment configs, restore pytest

- Rename L2_lingua_7b → L2_lingua_8b (model is Llama-3.1-8B)
- Rename L2_lingua_7b_mxfp8_qinit → L2_lingua_8b_mxfp8_qinit
- Keep only 4 example configs: 8b base, 8b mxfp8+qinit, 70b base, 70b mxfp8+qinit
- Remove 11 experiment-specific configs (bf16_baseline, fp8, mxfp8, thd, cp4, etc.)
- Restore pytest to requirements.txt (needed by CI runner)

Signed-off-by: Savitha Srinivasan <savithas@nvidia.com>

Author: savitha-eng