Compilation warnings

[wangkuiyi]

[16:59:53]	In file included from /paddle/paddle/optimizer/parameter_optimizer_test.cpp:21:0:
[16:59:53]	/paddle/build/third_party/install/gtest/include/gtest/gtest.h: In instantiation of 'testing::AssertionResult testing::internal::CmpHelperEQ(const char*, const char*, const T1&, const T2&) [with T1 = int; T2 = long unsigned int]':
[16:59:53]	/paddle/build/third_party/install/gtest/include/gtest/gtest.h:1421:23: required from 'static testing::AssertionResult testing::internal::EqHelper<lhs_is_null_literal>::Compare(const char*, const char*, const T1&, const T2&) [with T1 = int; T2 = long unsigned int; bool lhs_is_null_literal = false]'
[16:59:53]	/paddle/paddle/optimizer/parameter_optimizer_test.cpp:113:7: required from here
[16:59:53]	/paddle/build/third_party/install/gtest/include/gtest/gtest.h:1392:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
[16:59:53]	if (lhs == rhs) {
[16:59:53]