Upgrade pybind and refine pybind/protobuf.cc

[qingqing01]

Fix #7562

1. Upgrade pybind11 from v2.1.1 to v2.2.1 .
2. Remove the code about casting boost::variant for PyBind in pybind/protobuf.cc

[jacquesqiao]

what is the improvement of upgraded pybind11?

[qingqing01]

There are two reasons to upgrade pybind11:

1. The Refine profiler and expose to Python. #7576 needs add_ostream_redirect to redirect std::ostream to Python's streams . This must upgrade pybind11, the v2.1.1 does not support this feature.
2. As mentioned in Upgrade pybind and refine pybind/protobuf.cc #7562, the pybind11 v2.2.0 and v2.2.1 have supported to convert variant types, if upgrade pybind11, the following code in pybind/protobuf.cc can be removed.

Paddle/paddle/pybind/protobuf.cc

Lines 30 to 85 in 1511a04

	// Can be replaced by a generic lambda in C++14
	struct variant_caster_visitor : public boost::static_visitor<handle> {
	return_value_policy policy;
	handle parent;
	variant_caster_visitor(return_value_policy policy, handle parent)
	: policy(policy), parent(parent) {}
	template <class T>
	handle operator()(T const &src) const {
	return make_caster<T>::cast(src, policy, parent);
	}
	};
	template <class Variant>
	struct variant_caster;
	template <template <class...> class V, class... Ts>
	struct variant_caster<V<Ts...>> {
	using Type = V<Ts...>;
	template <typename T>
	typename std::enable_if<
	!std::is_same<T, boost::detail::variant::void_>::value, bool>::type
	try_load(handle src, bool convert) {
	auto caster = make_caster<T>();
	if (!load_success_ && caster.load(src, convert)) {
	load_success_ = true;
	value = cast_op<T>(caster);
	return true;
	}
	return false;
	}
	template <typename T>
	typename std::enable_if<std::is_same<T, boost::detail::variant::void_>::value,
	bool>::type
	try_load(handle src, bool convert) {
	return false;
	}
	bool load(handle src, bool convert) {
	auto unused = {false, try_load<Ts>(src, convert)...};
	(void)(unused);
	return load_success_;
	}
	static handle cast(Type const &src, return_value_policy policy,
	handle parent) {
	variant_caster_visitor visitor(policy, parent);
	return boost::apply_visitor(visitor, src);
	}
	PYBIND11_TYPE_CASTER(Type, _("Variant"));
	bool load_success_{false};
	};