Fix the curr_cond_batch_idx when --no_prior_preservation is not used

[wendashi]

If --no_prior_preservation is not used, there will be a problem. batch["input_ids"] becomes torch.Size([1, 77]). At this time, in batch["input_ids"][curr_cond_batch_idx], curr_cond_batch_idx=1. The following error occurred:

index 1 is out of bounds for dimension 0 with size 1
  File "/path/break-a-scene/train.py", line 1206, in main
    batch["input_ids"][curr_cond_batch_idx]
  File "/path/break-a-scene/train.py", line 628, in __init__
    self.main()
  File "/path/break-a-scene/train.py", line 1525, in <module>
    SpatialDreambooth()
  File "/path/break-a-scene_wenda/lib/python3.10/runpy.py", line 86, in _run_code
    exec(code, run_globals)
  File "/path/break-a-scene_wenda/lib/python3.10/runpy.py", line 196, in _run_module_as_main (Current frame)
    return _run_code(code, main_globals, None,
IndexError: index 1 is out of bounds for dimension 0 with size 1