Add OpProto implementation

OpProto is a proto message that helps 3rd-party language bindings, e.g.
`Python`, to generate operator creation methods. The operator creation
method is the low-level API for 3rd-party language bindings. Op creation
methods take the user's input in that language, and convert users inputs
into `OpDesc` message, then passing that `OpDesc` message to Paddle's
C++ core and create an operator.

* A separated `attr_type.proto` is added, because that file wound
  be included by `op_desc.proto` in future.

paddle/framework/CMakeLists.txt

@@ -5,3 +5,4 @@ nv_test(dim_test SRCS dim_test.cu DEPS ddim)
 cc_test(variable_test SRCS variable_test.cc)
 cc_test(scope_test SRCS scope_test.cc)
 cc_test(enforce_test SRCS enforce_test.cc)
+proto_library(op_proto SRCS op_proto.proto attr_type.proto)

paddle/framework/attr_type.proto

+/* Copyright (c) 2016 PaddlePaddle Authors. All Rights Reserve.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License. */
+
+syntax="proto2";
+package paddle.framework;
+
+// Attribute Type for paddle's Op.
+// Op contains many attributes. Each type of attributes could be different.
+// The AttrType will be shared between AttrDesc and AttrProto.
+enum AttrType {
+    INT = 0;
+    FLOAT = 1;
+    STRING = 2;
+    INTS = 3;
+    FLOATS = 4;
+    STRINGS = 5;
+}
\ No newline at end of file

paddle/framework/op_proto.proto

+/* Copyright (c) 2016 PaddlePaddle Authors. All Rights Reserve.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License. */
+
+// Protocol Message for 3rd-party language binding.
+//
+// Paddle Python package will use `OpProto` to generate op creation methods.
+// The op creation methods take user's input and generate `OpDesc` proto message,
+// then pass `OpDesc` to C++ side and create Op pointer.
+//
+syntax="proto2";
+package paddle.framework;
+
+import "attr_type.proto";
+
+// Attribute protocol message for 3rd-party language binding.
+// It will store the Op support what attribute and what type.
+message AttrProto {
+    // Supported attribute name. e.g. `scale` for cosine op.
+    required string name = 1;
+
+    // Supported attribute type.
+    required AttrType type = 2;
+
+    // Supported attribute comments. It helps 3rd-party language generate doc-string.
+    required string comment = 3;
+}
+
+// Input or output message for 3rd-party language binding.
+// It contains parameter name and its comments.
+message VarProto {
+    // Input or output name in that op creation function.
+    // e.g. `cos(a, b, output, ...)`, "a", "b", "output" are names.
+    required string name = 1;
+
+    // The comment for that input. It helps 3rd-party language generate doc-string.
+    required string comment = 2;
+}
+
+// Op protocol message for 3rd-party language binding.
+// It contains all information for generating op creation method.
+message OpProto {
+    // The input information to generate op creation method.
+    repeated VarProto inputs = 1;
+
+    // The output information to generate op creation method.
+    repeated VarProto outputs = 2;
+
+    // The attribute information to generate op creation method.
+    repeated AttrProto attrs = 3;
+
+    // The comments for that Op. It helps 3rd-party language generate
+    // doc-string. The whole documentation of that Op is generated by comment,
+    // inputs, outputs, attrs together.
+    required string comment = 4;
+
+    // The type of that Op.
+    required string type = 5;
+}