[`SigLIP`] Only import tokenizer if sentencepiece available (#28636)

Only import class if sp available

src/transformers/models/auto/tokenization_auto.py

@@ -379,7 +379,7 @@ else:
                     "SeamlessM4TTokenizerFast" if is_tokenizers_available() else None,
                 ),
             ),
-            ("siglip", ("SiglipTokenizer", None)),
+            ("siglip", ("SiglipTokenizer" if is_sentencepiece_available() else None, None)),
             ("speech_to_text", ("Speech2TextTokenizer" if is_sentencepiece_available() else None, None)),
             ("speech_to_text_2", ("Speech2Text2Tokenizer", None)),
             ("speecht5", ("SpeechT5Tokenizer" if is_sentencepiece_available() else None, None)),