[Pipeline Docs] Unconditional Latent Diffusion (#388)

* initial description

* add doc strings

docs/source/api/pipelines/latent_diffusion_uncond.mdx

-# GLIDE MODEL
+# Unconditional Latent Diffusion
\ No newline at end of file
+
+## Overview
+
+Unconditional Latent Diffusion was proposed in [High-Resolution Image Synthesis with Latent Diffusion Models](https://arxiv.org/abs/2112.10752) by Robin Rombach, Andreas Blattmann, Dominik Lorenz, Patrick Esser, Björn Ommer.
+
+The abstract of the paper is the following:
+
+*By decomposing the image formation process into a sequential application of denoising autoencoders, diffusion models (DMs) achieve state-of-the-art synthesis results on image data and beyond. Additionally, their formulation allows for a guiding mechanism to control the image generation process without retraining. However, since these models typically operate directly in pixel space, optimization of powerful DMs often consumes hundreds of GPU days and inference is expensive due to sequential evaluations. To enable DM training on limited computational resources while retaining their quality and flexibility, we apply them in the latent space of powerful pretrained autoencoders. In contrast to previous work, training diffusion models on such a representation allows for the first time to reach a near-optimal point between complexity reduction and detail preservation, greatly boosting visual fidelity. By introducing cross-attention layers into the model architecture, we turn diffusion models into powerful and flexible generators for general conditioning inputs such as text or bounding boxes and high-resolution synthesis becomes possible in a convolutional manner. Our latent diffusion models (LDMs) achieve a new state of the art for image inpainting and highly competitive performance on various tasks, including unconditional image generation, semantic scene synthesis, and super-resolution, while significantly reducing computational requirements compared to pixel-based DMs.*
+
+The original codebase can be found [here](https://github.com/CompVis/latent-diffusion).
+
+## Tips:
+
+- 
+- 
+- 
+
+## Available Pipelines:
+
+| Pipeline | Tasks | Colab
+|---|---|:---:|
+| [pipeline_latent_diffusion_uncond.py](https://github.com/huggingface/diffusers/blob/main/src/diffusers/pipelines/latent_diffusion_uncond/pipeline_latent_diffusion_uncond.py) | *Unconditional Image Generation* | - |
+
+## Examples:
+
+## API
+
+[[autodoc]] pipelines.latent_diffusion_uncond.pipeline_latent_diffusion_uncond.LDMPipeline
+    - __call__

src/diffusers/pipelines/latent_diffusion_uncond/pipeline_latent_diffusion_uncond.py

@@ -10,10 +10,17 @@ from ...schedulers import DDIMScheduler
 
 
 class LDMPipeline(DiffusionPipeline):
-
-    vqvae: VQModel
-    unet: UNet2DModel
-    scheduler: DDIMScheduler
+    r"""
+    This model inherits from [`DiffusionPipeline`]. Check the superclass documentation for the generic methods the
+    library implements for all the pipelines (such as downloading or saving, running on a particular device, etc.)
+
+    Parameters:
+        vqvae ([`VQModel`]):
+            Vector-quantized (VQ) Model to encode and decode images to and from latent representations.
+        unet ([`UNet2DModel`]): U-Net architecture to denoise the encoded image latents.
+        scheduler ([`SchedulerMixin`]):
+            [`DDIMScheduler`] is to be used in combination with `unet` to denoise the encoded image latens.
+    """
 
     def __init__(self, vqvae: VQModel, unet: UNet2DModel, scheduler: DDIMScheduler):
         super().__init__()
@@ -31,7 +38,23 @@ class LDMPipeline(DiffusionPipeline):
         return_dict: bool = True,
         **kwargs,
     ) -> Union[Tuple, ImagePipelineOutput]:
-        # eta corresponds to η in paper and should be between [0, 1]
+
+        r"""
+        Args:
+            batch_size (`int`, *optional*, defaults to 1):
+                Number of images to generate.
+            generator (`torch.Generator`, *optional*):
+                A [torch generator](https://pytorch.org/docs/stable/generated/torch.Generator.html) to make generation
+                deterministic.
+            num_inference_steps (`int`, *optional*, defaults to 50):
+                The number of denoising steps. More denoising steps usually lead to a higher quality image at the
+                expense of slower inference.
+            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 `nd.array`.
+            return_dict (`bool`, *optional*, defaults to `True`):
+                Whether or not to return a [`~pipeline_utils.ImagePipelineOutput`] instead of a plain tuple.
+        """
 
         if "torch_device" in kwargs:
             device = kwargs.pop("torch_device")