Adds TRANSFORMERS_TEST_DEVICE (#25506)

* Adds `TRANSFORMERS_TEST_DEVICE`
Mirrors the same API in the diffusers library. Useful in transformers
too.

* replace backend checking with trying `torch.device`

* Adds better error message for unknown test devices

* `make style`

* adds documentation showing `TRANSFORMERS_TEST_DEVICE` usage.

src/transformers/testing_utils.py

@@ -614,7 +614,16 @@ if is_torch_available():
     # Set env var CUDA_VISIBLE_DEVICES="" to force cpu-mode
     import torch
 
-    if torch.cuda.is_available():
+    if "TRANSFORMERS_TEST_DEVICE" in os.environ:
+        torch_device = os.environ["TRANSFORMERS_TEST_DEVICE"]
+        try:
+            # try creating device to see if provided device is valid
+            _ = torch.device(torch_device)
+        except RuntimeError as e:
+            raise RuntimeError(
+                f"Unknown testing device specified by environment variable `TRANSFORMERS_TEST_DEVICE`: {torch_device}"
+            ) from e
+    elif torch.cuda.is_available():
         torch_device = "cuda"
     elif _run_third_party_device_tests and is_torch_npu_available():
         torch_device = "npu"