fix: fix import handling of extra dependencies for chunking (#418)

Signed-off-by: Panos Vagenas <[email protected]>

Author: vagenas

docling_core/transforms/chunker/hybrid_chunker.py

@@ -9,7 +9,6 @@
 from typing import Any, Iterable, Iterator, Optional, Union
 
 from pydantic import BaseModel, ConfigDict, Field, computed_field, model_validator
-from transformers import PreTrainedTokenizerBase
 
 from docling_core.transforms.chunker.hierarchical_chunker import (
     ChunkingSerializerProvider,
@@ -18,6 +17,7 @@
 
 try:
     import semchunk
+    from transformers import PreTrainedTokenizerBase
 except ImportError:
     raise RuntimeError(
         "Extra required by module: 'chunking' by default (or 'chunking-openai' if "