[Docs] Adds a documentation page for evaluating diffusion models (#2516)

* add a documentation page for evaluating diffuion models.

* fix: checkpoint link.

* Apply suggestions from code review
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
Co-authored-by: Kashif Rasul <kashif.rasul@gmail.com>

* formatting fixes.

* formatting fixes.

* link to partiprompts dataset on hub.

* reflect on Pedro's comments.
Co-authored-by: Pedro <pedro@huggingface.co>

* Apply suggestions from code review
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>

* reflect on Pedro's comments.
Co-authored-by: Pedro <pedro@huggingface.co>

* update mention of FID.

* Apply suggestions from code review
Co-authored-by: Will Berman <wlbberman@gmail.com>
Co-authored-by: YiYi Xu <yixu310@gmail.com>

* minor nit.

* finish edges and add colab notebook.

* Apply suggestions from code review
Co-authored-by: Pedro Cuenca <pedro@huggingface.co>

* run formatting.

* additional feedback.

---------
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
Co-authored-by: Kashif Rasul <kashif.rasul@gmail.com>
Co-authored-by: Pedro <pedro@huggingface.co>
Co-authored-by: Will Berman <wlbberman@gmail.com>
Co-authored-by: YiYi Xu <yixu310@gmail.com>

docs/source/en/_toctree.yml

@@ -99,6 +99,8 @@
     title: How to contribute?
   - local: conceptual/ethical_guidelines
     title: Diffusers' Ethical Guidelines
+  - local: conceptual/evaluation
+    title: Evaluating Diffusion Models
   title: Conceptual Guides
 - sections:
   - sections: