Fix compiling warnings

[chengduoZH]

/paddle/Paddle/paddle/operators/math/sequence_padding_test.cc: In instantiation of ‘void TestSequencePadding(const LoD&, size_t) [with DeviceContext = paddle::platform::CPUDeviceContext; Place = paddle::platform::CPUPlace; T = float; paddle::framework::LoD = std::vector<thrust::host_vector<long unsigned int, thrust::system::cuda::experimental::pinned_allocator<long unsigned int> > >; size_t = long unsigned int]’: /paddle/Paddle/paddle/operators/math/sequence_padding_test.cc:84:66: required from here /paddle/Paddle/paddle/operators/math/sequence_padding_test.cc:34:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (size_t i = 0; i < cpu_seq.numel(); ++i) { ^ [ 87%] Generating paddle_fluid.dir/linear_chain_crf_op.objdir /paddle/Paddle/paddle/operators/math/sequence_padding_test.cc:72:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (size_t i = 0; i < cpu_seq.numel(); ++i) { ^ /paddle/Paddle/paddle/operators/math/sequence_padding_test.cc: In instantiation of ‘void TestSequencePadding(const LoD&, size_t) [with DeviceContext = paddle::platform::CUDADeviceContext; Place = paddle::platform::CUDAPlace; T = float; paddle::framework::LoD = std::vector<thrust::host_vector<long unsigned int, thrust::system::cuda::experimental::pinned_allocator<long unsigned int> > >; size_t = long unsigned int]’: /paddle/Paddle/paddle/operators/math/sequence_padding_test.cc:97:67: required from here /paddle/Paddle/paddle/operators/math/sequence_padding_test.cc:34:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (size_t i = 0; i < cpu_seq.numel(); ++i) { ^ /paddle/Paddle/paddle/operators/math/sequence_padding_test.cc:72:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (size_t i = 0; i < cpu_seq.numel(); ++i) { ^ 

[luotao1]

LGTM!