Ensure Flax pipeline always returns numpy array (#1435)

* Ensure Flax pipeline always returns numpy array.

* Clarify documentation.

src/diffusers/pipelines/stable_diffusion/__init__.py

@@ -63,15 +63,14 @@ if is_transformers_available() and is_flax_available():
         Output class for Stable Diffusion pipelines.
 
         Args:
-            images (`List[PIL.Image.Image]` or `np.ndarray`)
-                List of denoised PIL images of length `batch_size` or numpy array of shape `(batch_size, height, width,
-                num_channels)`. PIL images or numpy array present the denoised images of the diffusion pipeline.
+            images (`np.ndarray`)
+                Array of shape `(batch_size, height, width, num_channels)` with images from the diffusion pipeline.
             nsfw_content_detected (`List[bool]`)
                 List of flags denoting whether the corresponding generated image likely represents "not-safe-for-work"
                 (nsfw) content.
         """
 
-        images: Union[List[PIL.Image.Image], np.ndarray]
+        images: np.ndarray
         nsfw_content_detected: List[bool]
 
     from ...schedulers.scheduling_pndm_flax import PNDMSchedulerState

src/diffusers/pipelines/stable_diffusion/pipeline_flax_stable_diffusion.py

@@ -316,9 +316,6 @@ class FlaxStableDiffusionPipeline(FlaxDiffusionPipeline):
                 Pre-generated noisy latents, sampled from a Gaussian distribution, to be used as inputs for image
                 generation. Can be used to tweak the same generation with different prompts. If not provided, a latents
                 tensor will ge generated by sampling using the supplied random `generator`.
-            output_type (`str`, *optional*, defaults to `"pil"`):
-                The output format of the generate image. Choose between
-                [PIL](https://pillow.readthedocs.io/en/stable/): `PIL.Image.Image` or `np.array`.
             jit (`bool`, defaults to `False`):
                 Whether to run `pmap` versions of the generation and safety scoring functions. NOTE: This argument
                 exists because `__call__` is not yet end-to-end pmap-able. It will be removed in a future release.
@@ -382,6 +379,7 @@ class FlaxStableDiffusionPipeline(FlaxDiffusionPipeline):
 
             images = images.reshape(num_devices, batch_size, height, width, 3)
         else:
+            images = np.asarray(images)
             has_nsfw_concept = False
 
         if not return_dict: