Tweak badge link URL generation (#2677)

Summary:
Currently, the way feature badges are generated assumes that both documentations and the supported features page are on the same level from the root.

This does not work when we introduce `:autosummary:` which generates individual documentation pages one level below.

This commit changes it so that links to the supported features page are properly relative from the documentation level.

There is no appearance change from this commit.

Pull Request resolved: https://github.com/pytorch/audio/pull/2677

Reviewed By: carolineechen

Differential Revision: D39507451

Pulled By: mthrok

fbshipit-source-id: f18da4201f0eb747586be21c8bd9a958217aebc2

docs/source/custom_directives.py

 import hashlib
+import os
 from pathlib import Path
 from typing import List
 from urllib.parse import quote, urlencode
@@ -6,6 +7,7 @@ from urllib.parse import quote, urlencode
 import requests
 from docutils import nodes
 from docutils.parsers.rst.directives.images import Image
+from sphinx.util.docutils import SphinxDirective
 
 
 _THIS_DIR = Path(__file__).parent
@@ -38,10 +40,18 @@ def _fetch_image(url):
     path = _get_cache_path(url.encode("utf-8"), ext=".svg")
     if not path.exists():
         _download(url, path)
-    return str(path.relative_to(_THIS_DIR))
+    return os.sep + str(path.relative_to(_THIS_DIR))
 
 
-class BaseShield(Image):
+def _get_relpath(target, base):
+    target = os.sep + target
+    base = os.sep + base
+    target_path, filename = os.path.split(target)
+    rel_path = os.path.relpath(target_path, os.path.dirname(base))
+    return os.path.normpath(os.path.join(rel_path, filename))
+
+
+class BaseShield(Image, SphinxDirective):
     def run(self, params, alt, section) -> List[nodes.Node]:
         url = f"https://img.shields.io/static/v1?{urlencode(params, quote_via=quote)}"
         path = _fetch_image(url)
@@ -50,7 +60,8 @@ class BaseShield(Image):
         if "class" not in self.options:
             self.options["class"] = []
         self.options["class"].append("shield-badge")
-        self.options["target"] = f"supported_features.html#{section}"
+        target = _get_relpath("supported_features.html", self.env.docname)
+        self.options["target"] = f"{target}#{section}"
         return super().run()