Deprecate support for Transformers v4 (vllm-project#40389)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

Author: hmellor

vllm/transformers_utils/config.py

@@ -66,6 +66,13 @@
 
 logger = init_logger(__name__)
 
+if Version(version("transformers")) < Version("5.0.0"):
+    logger.warning(
+        "Support for Transformers v4 is deprecated. The Transformers v4 codepath will "
+        "become unmaintained in vLLM v0.22.0 and will be removed in vLLM v0.24.0. "
+        "Please upgrade to Transformers v5: pip install --upgrade transformers"
+    )
+
 
 class LazyConfigDict(dict):
     def __getitem__(self, key):