[SYMBOL] Add __iter__ and GetChildren for symbol (#268)

* [SYMBOL] Add __iter__ and GetChildren for symbol

* [SYMBOL] Fix lint

nnvm/include/nnvm/c_api.h

@@ -267,6 +267,14 @@ NNVM_DLL int NNSymbolGetNumOutputs(SymbolHandle symbol,
 NNVM_DLL int NNSymbolGetInternals(SymbolHandle symbol,
                                   SymbolHandle *out);
 /*!
+ * \brief Get a symbol that contains only direct children.
+ * \param symbol The symbol
+ * \param out The output symbol whose outputs are the direct children.
+ * \return 0 when success, -1 when failure happens
+ */
+NNVM_DLL int NNSymbolGetChildren(SymbolHandle symbol,
+                                 SymbolHandle *out);
+/*!
  * \brief Get index-th outputs of the symbol.
  * \param symbol The symbol
  * \param index the Index of the output.

nnvm/python/nnvm/symbol.py

@@ -143,6 +143,9 @@ class Symbol(SymbolBase):
             self.handle, _base.nn_uint(index), _ctypes.byref(handle)))
         return Symbol(handle=handle)
 
+    def __iter__(self):
+        return (self[i] for i in self.list_output_names())
+
     def attr(self, key):
         """Get attribute string from the symbol, this function only works for non-grouped symbol.
 
@@ -196,6 +199,17 @@ class Symbol(SymbolBase):
             self.handle, _ctypes.byref(handle)))
         return Symbol(handle=handle)
 
+    def get_children(self):
+        """Gets a new grouped symbol whose output contains
+           inputs to output nodes of the original symbol."""
+        handle = _base.SymbolHandle()
+        _check_call(_LIB.NNSymbolGetChildren(
+            self.handle, _ctypes.byref(handle)))
+        ret = Symbol(handle=handle)
+        if not ret.list_output_names():
+            return None
+        return ret
+
     def _get_list_copt(self, option):
         """internal function to get list option"""
         if option == 'all':

nnvm/src/c_api/c_api_symbolic.cc

@@ -141,6 +141,15 @@ int NNSymbolGetInternals(SymbolHandle symbol,
   API_END_HANDLE_ERROR(delete s);
 }
 
+int NNSymbolGetChildren(SymbolHandle symbol,
+                        SymbolHandle *out) {
+  Symbol *s = new Symbol();
+  API_BEGIN();
+  *s = static_cast<Symbol*>(symbol)->GetChildren();
+  *out = s;
+  API_END_HANDLE_ERROR(delete s);
+}
+
 int NNSymbolFree(SymbolHandle symbol) {
   API_BEGIN();
   delete static_cast<Symbol*>(symbol);