Fix the OpDesc construction in BlockDesc constructor

[kexinzhao]

Paddle/paddle/framework/block_desc.cc

Lines 164 to 166 in b15c675

	for (auto &op : other.ops_) {
	ops_.emplace_back(new OpDesc(*op, this));
	}

The code above is how the OpDesc is constructed when we call program.clone() on the python side.
This construction will call the following function:

Paddle/paddle/framework/op_desc.h

Lines 39 to 42 in b15c675

	OpDesc(const OpDesc &other, BlockDesc *block) {
	*this = other;
	block_ = block;
	}

Essentially, this calls a default assignment operator that copies the following data members:

Paddle/paddle/framework/op_desc.h

Lines 141 to 147 in b15c675

	proto::OpDesc desc_;
	BlockDesc *block_; // not_own
	// input arg name => input variable names
	VariableNameMap inputs_;
	// output arg name => output variable names
	VariableNameMap outputs_;
	AttributeMap attrs_;

Because the attribute type in the AttributeMap could be a BlockDesc*, cloning program desc in this way, makes for example the BLOCK attribute of the while operator in the copied program desc points to a BlockDesc in the original program desc.

This will be fixed along with #8161