Compile warning on sequence_padding

[Yancey0623]

In file included from /paddle/paddle/framework/ddim.h:21:0,
                 from /paddle/paddle/framework/lod_tensor.h:25,
                 from /paddle/paddle/operators/math/sequence_padding.h:17,
                 from /paddle/paddle/operators/math/sequence_padding.cc:15:
/paddle/paddle/operators/math/sequence_padding.cc: In instantiation of ‘void paddle::operators::math::PaddingLoDTensorFunctor<paddle::platform::CPUDeviceContext, T>::operator()(const paddle::platform::CPUDeviceContext&, const paddle::framework::LoDTensor&, paddle::framework::Tensor&, bool) [with T = float]’:
/paddle/paddle/operators/math/sequence_padding.cc:139:16:   required from here
/paddle/paddle/platform/enforce.h:227:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   __PADDLE_BINARY_COMPARE(__VAL0, __VAL1, ==, !=, __VA_ARGS__)
                                           ^
/paddle/paddle/platform/enforce.h:108:64: note: in definition of macro ‘UNLIKELY’
 #define UNLIKELY(condition) __builtin_expect(static_cast<bool>(condition), 0)
                                                                ^
/paddle/paddle/platform/enforce.h:227:3: note: in expansion of macro ‘__PADDLE_BINARY_COMPARE’
   __PADDLE_BINARY_COMPARE(__VAL0, __VAL1, ==, !=, __VA_ARGS__)
   ^
/paddle/paddle/operators/math/sequence_padding.cc:35:5: note: in expansion of macro ‘PADDLE_ENFORCE_EQ’
     PADDLE_ENFORCE_EQ(seq_dims[0], abs_offset_lod[level].back(),
     ^

[luotao1]

duplicated with #7408

[Yancey0623]

Thanks @luotao1, I will close this issue :)