[caches] Raise exception on offloaded static caches + multi device (#37974)

* skip tests on >1 gpu

* add todo

tests/utils/test_cache_utils.py

@@ -198,19 +198,24 @@ class CacheIntegrationTest(unittest.TestCase):
         )
         cls.model.config.sliding_window = 256  # hack to enable the use of caches with sliding windows
 
-    def _skip_on_uninstalled_cache_dependencies(self, cache_implementation):
-        """Function to skip tests on missing cache dependencies, given a cache implementation"""
+    def _skip_on_failed_cache_prerequisites(self, cache_implementation):
+        """Function to skip tests on failed cache prerequisites, given a cache implementation"""
+        # Installed dependencies
         if cache_implementation == "quantized" and not is_optimum_quanto_available():
             self.skipTest("Quanto is not available")
+        # Devices
         if "offloaded" in cache_implementation:
             has_accelerator = torch_device is not None and torch_device != "cpu"
             if not has_accelerator:
                 self.skipTest("Offloaded caches require an accelerator")
+            if cache_implementation in ["offloaded_static", "offloaded_hybrid_chunked"]:
+                if torch.cuda.device_count() != 1:
+                    self.skipTest("Offloaded static caches require exactly 1 GPU")
 
     @parameterized.expand(TEST_CACHE_IMPLEMENTATIONS)
     def test_cache_batched(self, cache_implementation):
         """Sanity check: caches' `.update` function expects batched inputs"""
-        self._skip_on_uninstalled_cache_dependencies(cache_implementation)
+        self._skip_on_failed_cache_prerequisites(cache_implementation)
 
         EXPECTED_GENERATION = ["A sequence: 1, 2, 3, 4, 5, 6, 7, 8,", "A sequence: A, B, C, D, E, F, G, H"]
 
@@ -239,7 +244,7 @@ class CacheIntegrationTest(unittest.TestCase):
         Sanity check: caches' `reorder_cache` is operational. We can confirm this by looking at the beam indices
         (an output sequence contains multiple beam indices).
         """
-        self._skip_on_uninstalled_cache_dependencies(cache_implementation)
+        self._skip_on_failed_cache_prerequisites(cache_implementation)
         if cache_implementation == "offloaded_hybrid_chunked":
             # TODO (joao, cyril): something is off with `offloaded_hybrid_chunked` aka `OffloadedHybridCache`: the
             # output sequence (and the corresponding beam scores, if we add `output_scores=True`) are significantly
@@ -273,7 +278,7 @@ class CacheIntegrationTest(unittest.TestCase):
     @parameterized.expand(TEST_CACHE_IMPLEMENTATIONS)
     def test_cache_extra_left_padding(self, cache_implementation):
         """Tests that adding extra left-padding does not affect the generation with the cache"""
-        self._skip_on_uninstalled_cache_dependencies(cache_implementation)
+        self._skip_on_failed_cache_prerequisites(cache_implementation)
 
         EXPECTED_GENERATION = ["The cat's whiskers are also a sign of anxiety."]