past_length is None

```
Traceback (most recent call last):
File "/root/miniconda3/lib/python3.10/site-packages/mmengine/runner/_flexible_runner.py", line 1271, in call_hook
getattr(hook, fn_name)(self, **kwargs)
File "/root/miniconda3/lib/python3.10/site-packages/xtuner/engine/hooks/evaluate_chat_hook.py", line 230, in before_train
self._generate_samples(runner, max_new_tokens=50)
File "/root/miniconda3/lib/python3.10/site-packages/xtuner/engine/hooks/evaluate_chat_hook.py", line 216, in _generate_samples
self._eval_images(runner, model, device, max_new_tokens,
File "/root/miniconda3/lib/python3.10/site-packages/xtuner/engine/hooks/evaluate_chat_hook.py", line 148, in _eval_images
generation_output = model.generate(
File "/root/miniconda3/lib/python3.10/site-packages/torch/utils/_contextlib.py", line 115, in decorate_context
return func(*args, **kwargs)
File "/root/miniconda3/lib/python3.10/site-packages/transformers/generation/utils.py", line 1758, in generate
result = self._sample(
File "/root/miniconda3/lib/python3.10/site-packages/transformers/generation/utils.py", line 2390, in _sample
model_kwargs = self._get_initial_cache_position(input_ids, model_kwargs)
File "/root/miniconda3/lib/python3.10/site-packages/transformers/generation/utils.py", line 1321, in _get_initial_cache_position
past_length = model_kwargs["past_key_values"][0][0].shape[2]
TypeError: 'NoneType' object is not subscriptable

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
File "/root/miniconda3/lib/python3.10/site-packages/xtuner/tools/train.py", line 360, in
main()
File "/root/miniconda3/lib/python3.10/site-packages/xtuner/tools/train.py", line 356, in main
runner.train()
File "/root/miniconda3/lib/python3.10/site-packages/mmengine/runner/_flexible_runner.py", line 1200, in train
model = self.train_loop.run() # type: ignore
File "/root/miniconda3/lib/python3.10/site-packages/mmengine/runner/loops.py", line 271, in run
self.runner.call_hook('before_train')
File "/root/miniconda3/lib/python3.10/site-packages/mmengine/runner/_flexible_runner.py", line 1273, in call_hook
raise TypeError(f'{e} in {hook}') from e
TypeError: 'NoneType' object is not subscriptable in
[2024-05-27 19:17:30,464] torch.distributed.elastic.multiprocessing.api: [ERROR] failed (exitcode: 1) local_rank: 0 (pid: 2618) of binary: /root/miniconda3/bin/python
Traceback (most recent call last):
File "/root/miniconda3/bin/torchrun", line 8, in
sys.exit(main())
File "/root/miniconda3/lib/python3.10/site-packages/torch/distributed/elastic/multiprocessing/errors/__init__.py", line 346, in wrapper
return f(*args, **kwargs)
File "/root/miniconda3/lib/python3.10/site-packages/torch/distributed/run.py", line 806, in main
run(args)
File "/root/miniconda3/lib/python3.10/site-packages/torch/distributed/run.py", line 797, in run
elastic_launch(
File "/root/miniconda3/lib/python3.10/site-packages/torch/distributed/launcher/api.py", line 134, in __call__
return launch_agent(self._config, self._entrypoint, list(args))
File "/root/miniconda3/lib/python3.10/site-packages/torch/distributed/launcher/api.py", line 264, in launch_agent
raise ChildFailedError(
torch.distributed.elastic.multiprocessing.errors.ChildFailedError:
============================================================
/root/miniconda3/lib/python3.10/site-packages/xtuner/tools/train.py FAILED
------------------------------------------------------------
Failures:

```
Any help would be greatly appreciated!