[Batched Generators] This PR adds generators that are useful to make batched generation fully reproducible (#1718)

* [Batched Generators] all batched generators

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* fix tests

* Apply suggestions from code review

Co-authored-by: Pedro Cuenca <pedro@huggingface.co>

* correct tests

Co-authored-by: Pedro Cuenca <pedro@huggingface.co>

docs/source/using-diffusers/reusing_seeds.mdx

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+
+# Re-using seeds for fast prompt engineering
+
+A common use case when generating images is to generate a batch of images, select one image and improve it with a better, more detailed prompt in a second run.
+To do this, one needs to make each generated image of the batch deterministic.
+Images are generated by denoising gaussian random noise which can be instantiated by passing a [torch generator](https://pytorch.org/docs/stable/generated/torch.Generator.html#generator).
+
+Now, for batched generation, we need to make sure that every single generated image in the batch is tied exactly to one seed. In 🧨 Diffusers, this can be achieved by not passing one `generator`, but a list 
+of `generators` to the pipeline.
+
+Let's go through an example using [`runwayml/stable-diffusion-v1-5`](runwayml/stable-diffusion-v1-5).
+We want to generate several versions of the prompt:
+
+```py
+prompt = "Labrador in the style of Vermeer"
+```
+
+Let's load the pipeline
+
+```python
+>>> from diffusers import DiffusionPipeline
+
+>>> pipe = DiffusionPipeline.from_pretrained("runwayml/stable-diffusion-v1-5", torch_dtype=torch.float16)
+>>> pipe = pipe.to("cuda")
+```
+
+Now, let's define 4 different generators, since we would like to reproduce a certain image. We'll use seeds `0` to `3` to create our generators.
+
+```python
+>>> import torch
+
+>>> generator = [torch.Generator(device="cuda").manual_seed(i) for i in range(4)]
+```
+
+Let's generate 4 images:
+
+```python
+>>> images = pipe(prompt, generator=generator, num_images_per_prompt=4).images
+>>> images
+```
+
+![img](https://huggingface.co/datasets/diffusers/diffusers-images-docs/resolve/main/reusabe_seeds.jpg)
+
+Ok, the last images has some double eyes, but the first image looks good! 
+Let's try to make the prompt a bit better **while keeping the first seed** 
+so that the images are similar to the first image.
+
+```python
+prompt = [prompt + t for t in [", highly realistic", ", artsy", ", trending", ", colorful"]]
+generator = [torch.Generator(device="cuda").manual_seed(0) for i in range(4)]
+```
+
+We create 4 generators with seed `0`, which is the first seed we used before.
+
+Let's run the pipeline again.
+
+```python
+>>> images = pipe(prompt, generator=generator).images
+>>> images
+```
+
+![img](https://huggingface.co/datasets/diffusers/diffusers-images-docs/resolve/main/reusabe_seeds_2.jpg)