Adds TRANSFORMERS_TEST_BACKEND (#25655)

* Adds `TRANSFORMERS_TEST_BACKEND`
Allows specifying arbitrary additional import following first `import torch`.
This is useful for some custom backends, that will require additional imports to trigger backend registration with upstream torch.
See https://github.com/pytorch/benchmark/pull/1805 for a similar change in `torchbench`.

* Update src/transformers/testing_utils.py

Co-authored-by: Yih-Dar <2521628+ydshieh@users.noreply.github.com>

* Adds real backend example to documentation

---------

Co-authored-by: Yih-Dar <2521628+ydshieh@users.noreply.github.com>

src/transformers/testing_utils.py

@@ -16,6 +16,7 @@ import collections
 import contextlib
 import doctest
 import functools
+import importlib
 import inspect
 import logging
 import multiprocessing
@@ -642,6 +643,17 @@ if is_torch_available():
         torch_device = "npu"
     else:
         torch_device = "cpu"
+
+    if "TRANSFORMERS_TEST_BACKEND" in os.environ:
+        backend = os.environ["TRANSFORMERS_TEST_BACKEND"]
+        try:
+            _ = importlib.import_module(backend)
+        except ModuleNotFoundError as e:
+            raise ModuleNotFoundError(
+                f"Failed to import `TRANSFORMERS_TEST_BACKEND` '{backend}'! This should be the name of an installed module. The original error (look up to see its"
+                f" traceback):\n{e}"
+            ) from e
+
 else:
     torch_device = None