[Feature] Import subpackge based on backend of dgl.nn (#1305)

* automatically select nn backend

* rename load_backend to private

* switch to backend_name
Co-authored-by: Jinjing Zhou <VoVAllen@users.noreply.github.com>

python/dgl/__init__.py

@@ -8,7 +8,6 @@ import socket
 from .backend import load_backend
 
 from . import function
-from . import nn
 from . import contrib
 from . import container
 from . import random

python/dgl/nn/__init__.py

 """Package for neural network common components."""
+import importlib
+import sys
+from ..backend import backend_name
+
+def _load_backend(mod_name):
+    mod = importlib.import_module('.%s' % mod_name, __name__)
+    thismod = sys.modules[__name__]
+    for api, obj in mod.__dict__.items():
+        setattr(thismod, api, obj)
+
+_load_backend(backend_name)