offline mode for firewalled envs (#10407)

* offline mode start

* add specific values

* fix fallback

* add test

* better values check and range

* test that actually works

* document the offline mode

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* more strict check

* cleaner test

* pt-only test

* style

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/modeling_utils.py

@@ -36,6 +36,7 @@ from .file_utils import (
     ModelOutput,
     cached_path,
     hf_bucket_url,
+    is_offline_mode,
     is_remote_url,
     replace_return_docstrings,
 )
@@ -964,6 +965,10 @@ class PreTrainedModel(nn.Module, ModuleUtilsMixin, GenerationMixin):
         revision = kwargs.pop("revision", None)
         mirror = kwargs.pop("mirror", None)
 
+        if is_offline_mode() and not local_files_only:
+            logger.info("Offline mode: forcing local_files_only=True")
+            local_files_only = True
+
         # Load config if we don't provide a configuration
         if not isinstance(config, PretrainedConfig):
             config_path = config if config is not None else pretrained_model_name_or_path