Reorder module import orders for dist-kvstore (#13742)

* Reorder module import orders for dist-kvstore

* more code comments

python/mxnet/__init__.py

@@ -54,7 +54,6 @@
 from . import lr_scheduler
 # use mx.kv as short for kvstore
 from . import kvstore as kv
-from . import kvstore_server
 # Runtime compile module
 from . import rtc
 # Attribute scope to add attributes to symbolic graphs
@@ -82,3 +81,11 @@
 from . import gluon
 
 __version__ = base.__version__
+
+# Dist kvstore module which launches a separate process when role is set to "server".
+# This should be done after other modules are initialized.
+# Otherwise this may result in errors when unpickling custom LR scheduler/optimizers.
+# For example, the LRScheduler in gluoncv depends on a specific version of MXNet, and
+# checks the __version__ attr of MXNet, which is not set on kvstore server due to the
+# fact that kvstore-server module is imported before the __version__ attr is set.
+from . import kvstore_server