Fix C++ API docs rendering. (#960)

Summary:
Pull Request resolved: #960

The CPP docs have extra top borders in functions and parameters. This commit fixes it and adjusts the rendering to be consistent with Python API documentation.

Reviewed By: mergennachin, dbort

Differential Revision: D50336290

fbshipit-source-id: 4f71579eeac29333b59e237efb7fcfeec9f92abe

Author: svekars

docs/source/_static/css/custom.css

@@ -160,3 +160,15 @@ details.sd-dropdown .highlight .n {
     clear: both;
   }
 }
+
+article.pytorch-article .class .method dt {
+    border-top: none;
+}
+
+article.pytorch-article .class .simple dt {
+    border-top: none;
+}
+
+article.pytorch-article .function dt.sig {
+    border-top: none;
+}