Simplify cache vars and allow for TRANSFORMERS_CACHE env (#4226)

* simplify cache vars and allow for TRANSFORMERS_CACHE env

As it currently stands, "TRANSFORMERS_CACHE" is not an accepted variable. It seems that the these variables were not updated when moving from version pytorch_transformers to transformers. In addition, the fallback procedure could be improved. and simplified. Pathlib seems redundant here.

* Update file_utils.py

src/transformers/file_utils.py

@@ -15,6 +15,7 @@ import tempfile
 from contextlib import contextmanager
 from functools import partial, wraps
 from hashlib import sha256
+from pathlib import Path
 from typing import Optional
 from urllib.parse import urlparse
 from zipfile import ZipFile, is_zipfile
@@ -68,19 +69,10 @@ except ImportError:
     )
 default_cache_path = os.path.join(torch_cache_home, "transformers")
 
-try:
-    from pathlib import Path
-
-    PYTORCH_PRETRAINED_BERT_CACHE = Path(
-        os.getenv("PYTORCH_TRANSFORMERS_CACHE", os.getenv("PYTORCH_PRETRAINED_BERT_CACHE", default_cache_path))
-    )
-except (AttributeError, ImportError):
-    PYTORCH_PRETRAINED_BERT_CACHE = os.getenv(
-        "PYTORCH_TRANSFORMERS_CACHE", os.getenv("PYTORCH_PRETRAINED_BERT_CACHE", default_cache_path)
-    )
 
-PYTORCH_TRANSFORMERS_CACHE = PYTORCH_PRETRAINED_BERT_CACHE  # Kept for backward compatibility
-TRANSFORMERS_CACHE = PYTORCH_PRETRAINED_BERT_CACHE  # Kept for backward compatibility
+PYTORCH_PRETRAINED_BERT_CACHE = os.getenv("PYTORCH_PRETRAINED_BERT_CACHE", default_cache_path)
+PYTORCH_TRANSFORMERS_CACHE = os.getenv("PYTORCH_TRANSFORMERS_CACHE", PYTORCH_PRETRAINED_BERT_CACHE)
+TRANSFORMERS_CACHE = os.getenv("TRANSFORMERS_CACHE", PYTORCH_TRANSFORMERS_CACHE)
 
 WEIGHTS_NAME = "pytorch_model.bin"
 TF2_WEIGHTS_NAME = "tf_model.h5"