export optim optimizers (#4822)

Co-authored-by: oneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>

docs/source/index.rst

@@ -35,6 +35,7 @@ OneFlow API Reference
     image
     train
     tmp
+    optim
 
 
 Indices and tables

docs/source/optim.rst

+oneflow.optim
+===================================
+Optimizers
+----------------------------------
+.. currentmodule:: oneflow.optim
+.. automodule:: oneflow.optim
+    :members:
+    :imported-members: