Replaces `DIFFUSERS_TEST_DEVICE` backend list with trying device (#4673)

This is a better method than comparing against a list of supported backends as it allows for supporting any number of backends provided they are installed on the user's system.
This should have no effect on the behaviour of tests in Huggingface's CI workers.
See transformers#25506 where this approach has already been added.

src/diffusers/utils/testing_utils.py

@@ -44,13 +44,13 @@ if is_torch_available():
 
     if "DIFFUSERS_TEST_DEVICE" in os.environ:
         torch_device = os.environ["DIFFUSERS_TEST_DEVICE"]
-
-        available_backends = ["cuda", "cpu", "mps"]
-        if torch_device not in available_backends:
-            raise ValueError(
-                f"unknown torch backend for diffusers tests: {torch_device}. Available backends are:"
-                f" {available_backends}"
-            )
+        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 `DIFFUSERS_TEST_DEVICE`: {torch_device}"
+            ) from e
         logger.info(f"torch_device overrode to {torch_device}")
     else:
         torch_device = "cuda" if torch.cuda.is_available() else "cpu"