fix _update_model_kwargs_for_generation

[Jintao-Huang]

What does this PR do?

The default value of model_inputs is None, and there is a line of code later on model_kwargs["cache_position"] = model_inputs.get("cache_position", None), which is unsafe. So, if the value of model_inputs is None, it should be set to {}.

When running the following code with the main branch code, an error occurs. However, it runs successfully using transformers==4.38.*.

# pip install ms-swift -U
import os

from swift.llm import (
    get_model_tokenizer, get_template, inference, ModelType,
    get_default_template_type, inference_stream
)
from swift.utils import seed_everything
import torch

model_type = ModelType.qwen1half_0_5b_chat
template_type = get_default_template_type(model_type)
print(f'template_type: {template_type}')  # template_type: qwen

kwargs = {}
# kwargs['use_flash_attn'] = True  # use flash_attn

model, tokenizer = get_model_tokenizer(model_type, torch.float16,
                                       model_kwargs={'device_map': 'auto'}, **kwargs)
template = get_template(template_type, tokenizer)
seed_everything(42)

model.generation_config.max_new_tokens = 128

query = 'hello'
gen = inference_stream(model, template, query)
print_idx = 0
print(f'query: {query}\nresponse: ', end='')
for response, history in gen:
    delta = response[print_idx:]
    print(delta, end='', flush=True)
    print_idx = len(response)
print()
print(f'history: {history}')

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• Did you write any new necessary tests?

Who can review?

@gante

[gante]

Hi @Jintao-Huang 👋

This need is not needed after #29467 gets merged in :) As such, I'm not approving/merging this PR so as to not generate conflicts