Speechllm develop gen duplex
@zhehuaichen Can you take a look? More details:
- Alignment: /home/kevinhu/s2s/s2s_salm_1ab_d1r_12fps_duplex4_data4b_mistral_1a_align.sh ([wandb](https://wandb.ai/aiapps/salm_s2s_huk/runs/tiqfqnt9?nw=nwuserhukevin))

- User text:
  * /home/kevinhu/s2s/s2s_salm_1ab_d1r_12fps_duplex4_data4b_mistral_1a_srctext.sh ([wandb](https://wandb.ai/aiapps/salm_s2s_huk/runs/m82t2sbp?nw=nwuserhukevin))
  * Compare to baseline seems bleu is a little better: https://wandb.ai/aiapps/salm_s2s_huk/reports/Ablation-Adding-user-text-prediction--VmlldzoxMDY2NDc0Ng All done - PTAL @zhehuaichen  Thanks for the review!
# What does this PR do ?

Add support for predicting user text and using word alignment.

Add flags
1. ++model.predict_source_text=True, for predicting user text
2. lhotse tag s2s_duplex_align, for use word alignment in target speech-text prediction