add auto_grown_mutex for selected rows

[jacquesqiao]

project: #10868

为什么mutex要放到 unique_ptr中？
https://stackoverflow.com/questions/30340029/copy-class-with-stdmutex
https://stackoverflow.com/questions/37172050/copy-or-move-constructor-for-a-class-with-a-member-stdmutex-or-other-non-copy

Paddle/paddle/fluid/operators/math/selected_rows_functor.h

Lines 70 to 86 in 654f5d3

	template <typename DeviceContext, typename T>
	struct Add {
	framework::SelectedRows operator()(const DeviceContext& context,
	const framework::SelectedRows& input1,
	const framework::SelectedRows& input2) {
	framework::SelectedRows out;
	out.set_rows(input1.rows());
	out.set_height(input1.height());
	out.mutable_value()->mutable_data<T>(input1.value().dims(),
	context.GetPlace());
	auto e_out = framework::EigenVector<T>::Flatten(*(out.mutable_value()));
	auto e_in1 = framework::EigenVector<T>::Flatten(input1.value());
	auto e_in2 = framework::EigenVector<T>::Flatten(input2.value());
	e_out.device(*context.eigen_device()) = e_in1 + e_in2;
	return out;
	}
	};

上面的代码，会在函数内部构造一个selected_rows, 然后return回来，如果selected rows里面有个成员是mutex，他不会生成一个默认的移动构造函数，编译上述functor的时候会挂，如果放到一个unique_prt里边，他会默认调用移动构造函数，把unique_ptr中的指针移动出来。

[Yancey1989]

LGTM

[Yancey1989]

Maybe we need a TODO here, make variable thread-safe completely.

[jacquesqiao]

will give a detailed explanation later.

[Yancey1989]

LGTM