* Let OpProto support multiple and temporary

* Each input/output of Paddle's Op could be a list. Add multiple mark to
  OpProto. Also add a `input_format`/`output_format` attribute if that
  Op has multiple input or output. The format of that attribute please
  reference the comments in `op_proto.proto`
* Add temporary mark, because some output of an Op is not used by user
  but used by other op for faster computation. Explicitly mark which
  output is temporary could let future memory/computation optimization.
* Add generated field to AttrProto.

* Add `AddInputs`/`AddOutputs` function

* It is more readable to invoke `AddInputs` not
  `AddInput(multiple=true)`.

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.