Fix sacremoses sof dependency for Transformers XL (#18321)

* Fix sacremoses sof dependency for Transofmers XL

* Add function to the submodule init

src/transformers/models/transfo_xl/tokenization_transfo_xl.py

@@ -27,10 +27,19 @@ from typing import List, Optional, Tuple
 
 import numpy as np
 
-import sacremoses as sm
-
 from ...tokenization_utils import PreTrainedTokenizer
-from ...utils import cached_path, is_torch_available, logging, torch_only_method
+from ...utils import (
+    cached_path,
+    is_sacremoses_available,
+    is_torch_available,
+    logging,
+    requires_backends,
+    torch_only_method,
+)
+
+
+if is_sacremoses_available():
+    import sacremoses as sm
 
 
 if is_torch_available():
@@ -187,6 +196,7 @@ class TransfoXLTokenizer(PreTrainedTokenizer):
             language=language,
             **kwargs,
         )
+        requires_backends(self, "sacremoses")
 
         if never_split is None:
             never_split = self.all_special_tokens

src/transformers/utils/__init__.py

@@ -109,6 +109,7 @@ from .import_utils import (
     is_pytesseract_available,
     is_pytorch_quantization_available,
     is_rjieba_available,
+    is_sacremoses_available,
     is_sagemaker_dp_enabled,
     is_sagemaker_mp_enabled,
     is_scatter_available,

src/transformers/utils/import_utils.py

@@ -444,6 +444,10 @@ def is_py3nvml_available():
     return importlib.util.find_spec("py3nvml") is not None
 
 
+def is_sacremoses_available():
+    return importlib.util.find_spec("sacremoses") is not None
+
+
 def is_apex_available():
     return importlib.util.find_spec("apex") is not None
 
@@ -799,6 +803,13 @@ PHONEMIZER_IMPORT_ERROR = """
 """
 
 
+# docstyle-ignore
+SACREMOSES_IMPORT_ERROR = """
+{0} requires the sacremoses library but it was not found in your environment. You can install it with pip:
+`pip install sacremoses`
+"""
+
+
 # docstyle-ignore
 SCIPY_IMPORT_ERROR = """
 {0} requires the scipy library but it was not found in your environment. You can install it with pip:
@@ -856,6 +867,7 @@ BACKENDS_MAPPING = OrderedDict(
         ("protobuf", (is_protobuf_available, PROTOBUF_IMPORT_ERROR)),
         ("pyctcdecode", (is_pyctcdecode_available, PYCTCDECODE_IMPORT_ERROR)),
         ("pytesseract", (is_pytesseract_available, PYTESSERACT_IMPORT_ERROR)),
+        ("sacremoses", (is_sacremoses_available, SACREMOSES_IMPORT_ERROR)),
         ("scatter", (is_scatter_available, SCATTER_IMPORT_ERROR)),
         ("pytorch_quantization", (is_pytorch_quantization_available, PYTORCH_QUANTIZATION_IMPORT_ERROR)),
         ("sentencepiece", (is_sentencepiece_available, SENTENCEPIECE_IMPORT_ERROR)),