Update sphinx requirement from ~=5.0 to ~=8.1 (#1304)

Summary:
Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a href="https://github.com/sphinx-doc/sphinx/releases">sphinx's releases</a>.</em></p>
<blockquote>
<h2>Sphinx 8.1.3</h2>
<p>Changelog: <a href="https://www.sphinx-doc.org/en/master/changes/8.1.html">https://www.sphinx-doc.org/en/master/changes/8.1.html</a></p>
<h2>Bugs fixed</h2>
<ul>
<li><a href="https://redirect.github.com/sphinx-doc/sphinx/issues/13013">#13013</a>: Restore support for <code>cut_lines()</code> with no object type. Patch by Adam Turner.</li>
</ul>
</blockquote>
</details>
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/sphinx-doc/sphinx/blob/v8.1.3/CHANGES.rst">sphinx's changelog</a>.</em></p>
<blockquote>
<h1>Release 8.1.3 (released Oct 13, 2024)</h1>
<h2>Bugs fixed</h2>
<ul>
<li><a href="https://redirect.github.com/sphinx-doc/sphinx/issues/13013">#13013</a>: Restore support for :func:<code>!cut_lines</code> with no object type.
Patch by Adam Turner.</li>
</ul>
<h1>Release 8.1.2 (released Oct 12, 2024)</h1>
<h2>Bugs fixed</h2>
<ul>
<li><a href="https://redirect.github.com/sphinx-doc/sphinx/issues/13012">#13012</a>: Expose :exc:<code>sphinx.errors.ExtensionError</code> in <code>sphinx.util</code>
for backwards compatibility.
This will be removed in Sphinx 9, as exposing the exception
in <code>sphinx.util</code> was never intentional.
:exc:<code>!ExtensionError</code> has been part of <code>sphinx.errors</code> since Sphinx 0.9.
Patch by Adam Turner.</li>
</ul>
<h1>Release 8.1.1 (released Oct 11, 2024)</h1>
<h2>Bugs fixed</h2>
<ul>
<li><a href="https://redirect.github.com/sphinx-doc/sphinx/issues/13006">#13006</a>: Use the preferred <a href="https://www.cve.org/">https://www.cve.org/</a> URL for
the :rst:role:<code>:cve: &lt;cve&gt;</code> role.
Patch by Hugo van Kemenade.</li>
<li><a href="https://redirect.github.com/sphinx-doc/sphinx/issues/13007">#13007</a>: LaTeX: Improve resiliency when the required
<code>fontawesome</code> or <code>fontawesome5</code> packages are not installed.
Patch by Jean-François B.</li>
</ul>
<h1>Release 8.1.0 (released Oct 10, 2024)</h1>
<h2>Dependencies</h2>
<ul>
<li><a href="https://redirect.github.com/sphinx-doc/sphinx/issues/12756">#12756</a>: Add lower-bounds to the <code>sphinxcontrib-*</code> dependencies.
Patch by Adam Turner.</li>
<li><a href="https://redirect.github.com/sphinx-doc/sphinx/issues/12833">#12833</a>: Update the LaTeX <code>parskip</code> package from 2001 to 2018.
Patch by Jean-François B.</li>
</ul>
<h2>Incompatible changes</h2>
<ul>
<li><a href="https://redirect.github.com/sphinx-doc/sphinx/issues/12763">#12763</a>: Remove unused internal class <code>sphinx.util.Tee</code>.</li>
</ul>

</blockquote>
<p>... (truncated)</p>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/a1510de4777eaa2e569435f95b05f6f3293d7035"><code>a1510de</code></a> Bump to 8.1.3 final</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/62e9606d63c8bbb4964213fd6b427d1483847662"><code>62e9606</code></a> Restore support for <code>cut_lines()</code> with no object type (<a href="https://redirect.github.com/sphinx-doc/sphinx/issues/13015">#13015</a>)</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/5ae32ce9bfe4a17a7f00e1e8d39a80449423c726"><code>5ae32ce</code></a> Bump version</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/a72b47bb408923cb7809eb9f96885545184e3773"><code>a72b47b</code></a> Bump to 8.1.2 final</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/39a45ad4073a4d8c3b7dfd64d22e8a88870dcc7c"><code>39a45ad</code></a> Expose <code>ExtensionError</code> in <code>sphinx.util</code> for backwards compatibility.</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/5a4859a2e489c66b38804e95bf77fd0baf4320dc"><code>5a4859a</code></a> Add docs about sphinx-autobuild (<a href="https://redirect.github.com/sphinx-doc/sphinx/issues/13011">#13011</a>)</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/05679efe7b34f8b2fb87605438c40248ac8cae83"><code>05679ef</code></a> Type-check the 'autodoc_intenum' example (<a href="https://redirect.github.com/sphinx-doc/sphinx/issues/12827">#12827</a>)</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/86d1d31fb370f031739079de7d827be0074e7661"><code>86d1d31</code></a> Prune CHANGES of unneeded sections</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/b6269d3790bb3bdd652ce67fecb59e6afddc8014"><code>b6269d3</code></a> Improve documentation for the Builder API (<a href="https://redirect.github.com/sphinx-doc/sphinx/issues/13008">#13008</a>)</li>
<li><a href="https://github.com/sphinx-doc/sphinx/commit/c46abc47210088a6c4fee9dac23badfcebc441d7"><code>c46abc4</code></a> Improve clarity for <code>master_doc</code> and <code>root_doc</code></li>
<li>Additional commits viewable in <a href="https://github.com/sphinx-doc/sphinx/compare/v5.0.0...v8.1.3">compare view</a></li>
</ul>
</details>
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</details>


Reviewed By: kiukchung

Differential Revision: D108447591

Pulled By: kiukchung

Author: dependabot[bot]

docs/requirements.txt

@@ -1,4 +1,4 @@
-sphinx~=5.0
+sphinx~=8.1
 sphinxcontrib-katex
 pytorch-sphinx-theme @ git+https://github.com/pytorch/pytorch_sphinx_theme.git
 jinja2>=3.1.4