fix diffusion tests ci

tests/policies/test_policies.py

@@ -43,12 +43,21 @@ from lerobot.policies.vqbet.configuration_vqbet import VQBeTConfig
 from lerobot.policies.vqbet.modeling_vqbet import VQBeTHead
 from lerobot.utils.constants import ACTION, OBS_IMAGES, OBS_STATE
 from lerobot.utils.feature_utils import dataset_to_policy_features
+from lerobot.utils.import_utils import is_package_available
 from lerobot.utils.random_utils import seeded_context
 from lerobot.utils.utils import cycle
 from tests.artifacts.policies.save_policy_to_safetensors import get_policy_stats
 from tests.utils import DEVICE, require_cpu, require_env, require_x86_64_kernel
 
-AVAILABLE_POLICIES = ["act", "diffusion", "tdmpc", "vqbet"]
+# Policies that require optional heavy dependencies to instantiate
+_POLICY_REQUIRED_PACKAGES: dict[str, tuple[str, ...]] = {
+    "diffusion": ("diffusers",),
+}
+
+_ALL_POLICIES = ["act", "diffusion", "tdmpc", "vqbet"]
+AVAILABLE_POLICIES = [
+    p for p in _ALL_POLICIES if all(is_package_available(pkg) for pkg in _POLICY_REQUIRED_PACKAGES.get(p, ()))
+]
 
 
 @pytest.fixture
@@ -344,7 +353,7 @@ def test_multikey_construction(multikey: bool):
         # to normalize the image at all. In our current codebase we dont normalize at all. But there is still a minor difference
         # that fails the test. However, by testing to normalize the image with 0.5 0.5 in the current codebase, the test pass.
         # Thus, we deactivate this test for now.
-        (
+        pytest.param(
             "lerobot/pusht",
             "diffusion",
             {
@@ -353,6 +362,7 @@ def test_multikey_construction(multikey: bool):
                 "down_dims": [128, 256, 512],
             },
             "",
+            marks=pytest.mark.skipif(not is_package_available("diffusers"), reason="diffusers not installed"),
         ),
         ("lerobot/aloha_sim_insertion_human", "act", {"n_action_steps": 10}, ""),
         (