[Backend] backend interface (dmlc#109)

* backend interface

* small fix

* more comments to the data type dict

* WIP

* convert_to and narrow

* WIP

* pytorch and numpy backend; WIP on mxnet backend

* mxnet backend

* narrow

* Fix all usages

* fix for mx

* fix for mx

* fix mx

* fix mx

* fix mx

* fix mx

* fix mx

* fix mx

* fix mx

* revert jenkins

* add sparse_matrix api

* sparse matrix api

* some fixme

* Fix as requested

python/dgl/backend/__init__.py

@@ -1,16 +1,53 @@
 from __future__ import absolute_import
 
-import os
-
-__backend__ = os.environ.get('DGLBACKEND', 'pytorch').lower()
-if __backend__ == 'numpy':
-    from .numpy import *
-    create_immutable_graph_index=None
-elif __backend__ == 'pytorch':
-    from .pytorch import *
-    create_immutable_graph_index=None
-elif __backend__ == 'mxnet':
-    from .mxnet import *
-    from .mxnet_immutable_graph_index import create_immutable_graph_index
-else:
-    raise Exception("Unsupported backend %s" % __backend__)
+import sys, os
+import importlib
+
+from . import backend
+
+_enabled_apis = set()
+
+def _gen_missing_api(api, mod_name):
+    def _missing_api(*args, **kwargs):
+        raise ImportError('API "%s" is not supported by backend "%s".'
+                          ' You can switch to other backends by setting'
+                          ' the DGLBACKEND environment.' % (api, mod_name))
+    return _missing_api
+
+def _load_backend():
+    mod_name = os.environ.get('DGLBACKEND', 'pytorch').lower()
+    mod = importlib.import_module('.%s' % mod_name, __name__)
+    thismod = sys.modules[__name__]
+    for api in backend.__dict__.keys():
+        if api == 'data_type_dict':
+            # load data type
+            if api not in mod.__dict__:
+                raise ImportError('API "data_type_dict" is required but missing for'
+                                  ' backend "%s".' % (mod_name))
+            data_type_dict = mod.__dict__[api]()
+            for name, dtype in data_type_dict.items():
+                setattr(thismod, name, dtype)
+        else:
+            # load functions
+            if api in mod.__dict__:
+                _enabled_apis.add(api)
+                setattr(thismod, api, mod.__dict__[api])
+            else:
+                setattr(thismod, api, _gen_missing_api(api, mod_name))
+
+_load_backend()
+
+def is_enabled(api):
+    """Return true if the api is enabled by the current backend.
+
+    Parameters
+    ----------
+    api : str
+        The api name.
+
+    Returns
+    -------
+    bool
+        True if the API is enabled by the current backend.
+    """
+    return api in _enabled_apis