add cpu_pinned in __init__ (apache#11030)

This make mx.cpu_pinned can be used.

python/mxnet/__init__.py

@@ -21,7 +21,7 @@
 """MXNet: a concise, fast and flexible framework for deep learning."""
 from __future__ import absolute_import
 
-from .context import Context, current_context, cpu, gpu
+from .context import Context, current_context, cpu, gpu, cpu_pinned
 from . import engine
 from .base import MXNetError
 from . import base