Rename dygraph parallel env & add doc (#22925)

* add dygraph parallel env doc, test=develop

* polish details, test=develop, test=document_fix

* fix examples error in other apis, test=develop

* fix more example error in other api, test=develop

* add white list for gpu examples, test=develop, test=document_fix

tools/wlist.json

@@ -310,6 +310,10 @@
         "Recall.eval",
         "FC.forward",
         "While.block",
-        "DGCMomentumOptimizer"
+        "DGCMomentumOptimizer",
+        "ParallelEnv",
+        "DataParallel",
+        "DataParallel.scale_loss",
+        "DataParallel.apply_collective_grads"
     ]
 }