Fix Wan and Motif video pipeline fast test failures (#14269)

* Fix Wan and Motif video pipeline fast test failures

Description

This PR fixes several failing fast tests for Motif Video and Wan pipelines.

Changes

Updated test_motif_video.py to use a Motif-specific fp16 save/load tolerance.
T5Gemma2Encoder rebuilds non-persistent RoPE buffers after save/load, which introduces small fp16 numerical drift. The test now keeps coverage while allowing this expected drift.

Updated transformer_wan.py to keep Wan RoPE modules in fp32 during mixed precision handling.
This avoids fp16 precision drift in Wan save/load paths.

Updated memory.py group offloading coverage to include transformer_2 and skip None optional components.
This fixes Wan 2.2 memory/offloading tests where optional secondary transformers may be absent.

Updated stale expected slices in test_wan_image_to_video.py for:

TestWanImageToVideoPipeline::test_inference
TestWanFLFToVideoPipeline::test_inference

Signed-off-by: Wang, Yi A <yi.a.wang@intel.com>

* revert change in test_wan_image_to_video.py

Signed-off-by: Wang, Yi A <yi.a.wang@intel.com>

---------

Signed-off-by: Wang, Yi A <yi.a.wang@intel.com>
Co-authored-by: Sayak Paul <spsayakpaul@gmail.com>

Author: sywangyi

src/diffusers/models/transformers/transformer_wan.py

@@ -546,7 +546,7 @@ class WanTransformer3DModel(
     _supports_gradient_checkpointing = True
     _skip_layerwise_casting_patterns = ["patch_embedding", "condition_embedder", "norm"]
     _no_split_modules = ["WanTransformerBlock"]
-    _keep_in_fp32_modules = ["time_embedder", "scale_shift_table", "norm1", "norm2", "norm3"]
+    _keep_in_fp32_modules = ["rope", "time_embedder", "scale_shift_table", "norm1", "norm2", "norm3"]
     _keys_to_ignore_on_load_unexpected = ["norm_added_q"]
     _repeated_blocks = ["WanTransformerBlock"]
     _cp_plan = {

tests/pipelines/motif_video/test_motif_video.py

@@ -141,3 +141,7 @@ def test_inference(self):
         generated_video = video[0]
 
         self.assertEqual(generated_video.shape, (9, 16, 16, 3))
+
+    def test_save_load_float16(self):
+        # T5Gemma2Encoder rebuilds non-persistent RoPE buffers after save/load, which causes small fp16 drift.
+        super().test_save_load_float16(expected_max_diff=5e-2)

tests/pipelines/testing_utils/memory.py

@@ -288,12 +288,15 @@ def enable_group_offload_on_component(pipe, group_offloading_kwargs):
                 "text_encoder_2",
                 "text_encoder_3",
                 "transformer",
+                "transformer_2",
                 "unet",
                 "controlnet",
             ]:
                 if not hasattr(pipe, component_name):
                     continue
                 component = getattr(pipe, component_name)
+                if component is None:
+                    continue
                 if not getattr(component, "_supports_group_offloading", True):
                     continue
                 if hasattr(component, "enable_group_offload"):