Switch to functional backend lazily

After this commit, instead of loading the libraries ahead of usage,
when an arrayfire function is called for first time, a functional
backend is loaded on the fly.

Author: 9prady9

arrayfire/library.py

@@ -440,10 +440,30 @@ def set_unsafe(self, name):
             raise RuntimeError("Backend not found")
         self.__name = name
 
-    def __init__(self):
-
+    def _loadlibs(self):
+        """
+        function that loads ArrayFire upstream libraries
+        """
         more_info_str = "Please look at https://github.com/arrayfire/arrayfire-python/wiki for more information."
 
+        # Iterate in reverse order of preference
+        for name in ('cpu', 'opencl', 'cuda', ''):
+            libnames = self.__libname(name)
+            for libname in libnames:
+                try:
+                    ct.cdll.LoadLibrary(libname)
+                    __name = 'unified' if name == '' else name
+                    self.__clibs[__name] = ct.CDLL(libname)
+                    self.__name = __name
+                    break;
+                except:
+                    pass
+
+        if (self.__name is None):
+            raise RuntimeError("Could not load any ArrayFire libraries.\n" +
+                               more_info_str)
+
+    def __init__(self):
         pre, post, AF_PATH, CUDA_FOUND = _setup()
 
         self.__pre = pre
@@ -468,7 +488,6 @@ def __init__(self):
                                    'cpu'     : 1,
                                    'cuda'    : 2,
                                    'opencl'  : 4}
-
         # Try to pre-load forge library if it exists
         libnames = self.__libname('forge', '')
         for libname in libnames:
@@ -477,30 +496,15 @@ def __init__(self):
             except:
                 pass
 
-        # Iterate in reverse order of preference
-        for name in ('cpu', 'opencl', 'cuda', ''):
-            libnames = self.__libname(name)
-            for libname in libnames:
-                try:
-                    ct.cdll.LoadLibrary(libname)
-                    __name = 'unified' if name == '' else name
-                    self.__clibs[__name] = ct.CDLL(libname)
-                    self.__name = __name
-                    break;
-                except:
-                    pass
-
-        if (self.__name is None):
-            raise RuntimeError("Could not load any ArrayFire libraries.\n" +
-                               more_info_str)
-
     def get_id(self, name):
         return self.__backend_name_map[name]
 
     def get_name(self, bk_id):
         return self.__backend_map[bk_id]
 
     def get(self):
+        if (self.__clibs[self.__name] is None):
+            self._loadlibs()
         return self.__clibs[self.__name]
 
     def name(self):