use cff standard for citation information (pytorch#86200)

GH picks up on our `CITATION` file in the root of the repository.

![Screenshot from 2022-10-04 11-34-54](https://user-images.githubusercontent.com/6849766/193811617-b71ef606-a043-498b-bb2d-14b6c05e79e7.png)

However, [the preferred way](https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files) is use a `CITATION.cff` file instead since GH supports the [citation file format (CFF) standard](https://github.com/citation-file-format/citation-file-format). With this PR, the prompt changes to

![Screenshot from 2022-10-04 13-48-21](https://user-images.githubusercontent.com/6849766/193812010-026bfad7-7c4e-4b59-a90a-1d3ad47303d0.png)

with the following auto-generated bibtex entry:

```bibtex
@inproceedings{Paszke_PyTorch_An_Imperative_2019,
author = {Paszke, Adam and Gross, Sam and Massa, Francisco and Lerer, Adam and Bradbury, James and Chanan, Gregory and Killeen, Trevor and Lin, Zeming and Gimelshein, Natalia and Antiga, Luca and Desmaison, Alban and Kopf, Andreas and Yang, Edward and DeVito, Zachary and Raison, Martin and Tejani, Alykhan and Chilamkurthy, Sasank and Steiner, Benoit and Fang, Lu and Bai, Junjie and Chintala, Soumith},
booktitle = {Advances in Neural Information Processing Systems 32},
pages = {8024--8035},
publisher = {Curran Associates, Inc.},
title = {{PyTorch: An Imperative Style, High-Performance Deep Learning Library}},
url = {http://papers.neurips.cc/paper/9015-pytorch-an-imperative-style-high-performance-deep-learning-library.pdf},
year = {2019}
}
```

Comparing with what we currently have the only significant difference is that the editors are no longer listed although the metadata is there. This is an issue with GH's automatic conversion and might be fixed in the future. Plus, the cite key was changed from `NEURIPS2019_9015` to `Paszke_PyTorch_An_Imperative_2019`, but this has no effect on the rendered result.

Do we also want to adopt the CFF standard?
Pull Request resolved: pytorch#86200
Approved by: https://github.com/dagitses

CITATION.cff

@@ -0,0 +1,73 @@
+cff-version: 1.2.0
+message: If you use this software, please cite it as below.
+title: PyTorch
+authors:
+  - family-names: PyTorch Team
+url: https://pytorch.org
+preferred-citation:
+  type: conference-paper
+  title: "PyTorch: An Imperative Style, High-Performance Deep Learning Library"
+  authors:
+    - family-names: Paszke
+      given-names: Adam
+    - family-names: Gross
+      given-names: Sam
+    - family-names: Massa
+      given-names: Francisco
+    - family-names: Lerer
+      given-names: Adam
+    - family-names: Bradbury
+      given-names: James
+    - family-names: Chanan
+      given-names: Gregory
+    - family-names: Killeen
+      given-names: Trevor
+    - family-names: Lin
+      given-names: Zeming
+    - family-names: Gimelshein
+      given-names: Natalia
+    - family-names: Antiga
+      given-names: Luca
+    - family-names: Desmaison
+      given-names: Alban
+    - family-names: Kopf
+      given-names: Andreas
+    - family-names: Yang
+      given-names: Edward
+    - family-names: DeVito
+      given-names: Zachary
+    - family-names: Raison
+      given-names: Martin
+    - family-names: Tejani
+      given-names: Alykhan
+    - family-names: Chilamkurthy
+      given-names: Sasank
+    - family-names: Steiner
+      given-names: Benoit
+    - family-names: Fang
+      given-names: Lu
+    - family-names: Bai
+      given-names: Junjie
+    - family-names: Chintala
+      given-names: Soumith
+  collection-title: Advances in Neural Information Processing Systems 32
+  collection-type: proceedings
+  editors:
+    - family-names: Wallach
+      given-names: H.
+    - family-names: Larochelle
+      given-names: H.
+    - family-names: Beygelzimer
+      given-names: A.
+    - family-names: "d'Alché-Buc"
+      given-names: F.
+    - family-names: Fox
+      given-names: E.
+    - family-names: Garnett
+      given-names: R.
+  start: 8024
+  end: 8035
+  year: 2019
+  publisher:
+    name: Curran Associates, Inc.
+  url: http://papers.neurips.cc/paper/9015-pytorch-an-imperative-style-high-performance-deep-learning-library.pdf