[tests] test `encode_prompt()` in isolation (#10438)

* poc encode_prompt() tests

* fix

* updates.

* fixes

* fixes

* updates

* updates

* updates

* revert

* updates

* updates

* updates

* updates

* remove SDXLOptionalComponentsTesterMixin.

* remove tests that directly leveraged encode_prompt() in some way or the other.

* fix imports.

* remove _save_load

* fixes

* fixes

* fixes

* fixes

tests/pipelines/wuerstchen/test_wuerstchen_decoder.py

@@ -186,3 +186,7 @@ class WuerstchenDecoderPipelineFastTests(PipelineTesterMixin, unittest.TestCase)
     @unittest.skip(reason="bf16 not supported and requires CUDA")
     def test_float16_inference(self):
         super().test_float16_inference()
+
+    @unittest.skip("Test not supoorted.")
+    def test_encode_prompt_works_in_isolation(self):
+        super().test_encode_prompt_works_in_isolation()

tests/pipelines/wuerstchen/test_wuerstchen_prior.py

@@ -267,3 +267,7 @@ class WuerstchenPriorPipelineFastTests(PipelineTesterMixin, unittest.TestCase):
         lora_image_embed = output_lora.image_embeddings
 
         self.assertTrue(image_embed.shape == lora_image_embed.shape)
+
+    @unittest.skip("Test not supported as dtype cannot be inferred without the text encoder otherwise.")
+    def test_encode_prompt_works_in_isolation(self):
+        pass