Remove mandatory dependency of tokenizers

sentence_transformers/models/StaticEmbedding.py

@@ -8,12 +8,14 @@
 import torch
 from safetensors.torch import load_file as load_safetensors_file
 from safetensors.torch import save_file as save_safetensors_file
-from tokenizers import Tokenizer
 from torch import nn
-from transformers import PreTrainedTokenizerFast
+from transformers import PreTrainedTokenizerFast, is_tokenizers_available, requires_backends
 
 from sentence_transformers.util import get_device_name
 
+if is_tokenizers_available():
+    from tokenizers import Tokenizer
+
 
 class StaticEmbedding(nn.Module):
     def __init__(
@@ -60,6 +62,7 @@ def __init__(
             ValueError: If neither `embedding_weights` nor `embedding_dim` is provided.
         """
         super().__init__()
+        requires_backends(self, "tokenizers")
 
         if isinstance(tokenizer, PreTrainedTokenizerFast):
             tokenizer = tokenizer._tokenizer
@@ -118,6 +121,7 @@ def save(self, save_dir: str, safe_serialization: bool = True, **kwargs) -> None
         self.tokenizer.save(str(Path(save_dir) / "tokenizer.json"))
 
     def load(load_dir: str, **kwargs) -> StaticEmbedding:
+        requires_backends(StaticEmbedding, "tokenizers")
         tokenizer = Tokenizer.from_file(str(Path(load_dir) / "tokenizer.json"))
         if os.path.exists(os.path.join(load_dir, "model.safetensors")):
             weights = load_safetensors_file(os.path.join(load_dir, "model.safetensors"))
@@ -157,7 +161,7 @@ def from_distillation(
         Raises:
             ImportError: If the `model2vec` package is not installed.
         """
-
+        requires_backends(cls, "tokenizers")
         try:
             from model2vec import distill
         except ImportError:
@@ -193,7 +197,7 @@ def from_model2vec(cls, model_id_or_path: str) -> StaticEmbedding:
         Raises:
             ImportError: If the `model2vec` package is not installed.
         """
-
+        requires_backends(cls, "tokenizers")
         try:
             from model2vec import StaticModel
         except ImportError: