在使用BERTopic.merge_models后，我无法保存生成的模型。以下是一些重现代码：

import bertopic
from bertopic import BERTopic
from sklearn.datasets import fetch_20newsgroups
from sklearn.decomposition import PCA

print(f'Using bertopic version {bertopic.__version__}')

embedding_model = 'all-mpnet-base-v2'

def new_topic_model():
    return BERTopic(
        embedding_model=embedding_model,
        # Using PCA because UMAP segfaults on my machine
        umap_model=PCA(n_components=10, random_state=40),
    )

docs = fetch_20newsgroups(subset='all',  remove=('headers', 'footers', 'quotes'))['data']
docs_a = docs[:400]
docs_b = docs[400:500]

topic_model_a = new_topic_model()
topic_model_a.fit(docs_a)
# No problem saving topic_model_a
topic_model_a.save(
    './topic_model_a',
    serialization='safetensors',
    save_embedding_model=embedding_model,
    save_ctfidf=True,
)

topic_model_b = new_topic_model()
topic_model_b.fit(docs_b)
# No problem saving topic_model_b
topic_model_b.save(
    './topic_model_b',
    serialization='safetensors',
    save_embedding_model=embedding_model,
    save_ctfidf=True,
)

topic_model = BERTopic.merge_models([topic_model_a, topic_model_b])
# Problem here: AttributeError: 'NoneType' object has no attribute 'indptr'
topic_model.save(
    './topic_model',
    serialization='safetensors',
    save_embedding_model=embedding_model,
    save_ctfidf=True,
)

我得到以下输出：

% python3 repro.py
Using bertopic version 0.16.2
Traceback (most recent call last):
  File "[...]/BERTopic/repro.py", line 43, in <module>
    topic_model.save(
  File "[...]/BERTopic/bertopic/_bertopic.py", line 3112, in save
    save_utils.save_ctfidf(model=self, save_directory=save_directory, serialization=serialization)
  File "[...]/BERTopic/bertopic/_save_utils.py", line 318, in save_ctfidf
    indptr = torch.from_numpy(model.c_tf_idf_.indptr)
                              ^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'indptr'