Use sphinx-galery 0.9 and simplify conf.py (#3724)

docs/requirements.txt

 sphinx==2.4.4
-sphinx-gallery
+sphinx-gallery>=0.9.0
 matplotlib
 numpy
 -e git+git://github.com/pytorch/pytorch_sphinx_theme.git#egg=pytorch_sphinx_theme

docs/source/conf.py

@@ -281,15 +281,6 @@ def inject_minigalleries(app, what, name, obj, options, lines):
     https://www.sphinx-doc.org/en/master/usage/extensions/autodoc.html
     """
 
-    conf_file_path = Path(__file__).parent.absolute()
-    backrefs_path = conf_file_path / sphinx_gallery_conf['backreferences_dir'] / (name + '.examples')
-    if not (os.path.isfile(backrefs_path) and os.path.getsize(backrefs_path) > 0):
-        # We avoid showing the (empty) minigallery if there's nothing to show, i.e. if the object
-        # isn't used in any example.
-        # FIXME: this check can be removed once https://github.com/sphinx-gallery/sphinx-gallery/pull/813
-        # is merged and the new sphinx-gallery version (> 0.8.x) is released.
-        return
-
     if what in ("class", "function"):
         lines.append(f".. minigallery:: {name}")
         lines.append(f"    :add-heading: Examples using ``{name.split('.')[-1]}``:")