Enhance ProgramDesc constructor taking protobuf string for RNN

[kexinzhao]

Right now, when constructing a ProgramDesc instance from a proto::ProgramDesc, there will be error for test_rnn_encoder_decoder.

The reason is the following code:

Paddle/paddle/framework/op_desc.cc

Lines 121 to 129 in 445c74c

	for (const proto::OpDesc::Attr &attr : desc_.attrs()) {
	std::string attr_name = attr.name();
	if (attr.type() != proto::AttrType::BLOCK) {
	attrs_[attr_name] = GetAttrValue(attr);
	} else {
	auto bid = attr.block_idx();
	attrs_[attr_name] = prog->MutableBlock(bid);
	}
	}

where it tries to assign a block pointer that hasn't been created yet (because the ProgramDesc constructor calls the BlockDesc constructor one by one following the order of the block idx) as a attribute to a RNN while operator.

This is issue will be fixed together with #8062 and #8059

[sidgoyal78]

I think this is also relevant to the Machine translation chapter (Issue #7999). I get weird and similar outputs when i try to save the model for the NMT chapter.