fix msan in vec_reduce_all (#37853)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37853
```
Uninitialized value was created by an allocation of 'acc_arr_next' in the stack frame of function '_ZN2at6vec25614vec_reduce_allIfZZNS_6native12_GLOBAL__N_124_vec_log_softmax_lastdimIfEEvPT_S6_llENKUlllE_clEllEUlRNS0_12_GLOBAL__N_16Vec256IfEESB_E_EES5_RKT0_NS9_IS5_EEl'
#0 0xa961530 in float at::vec256::vec_reduce_all<float, void at::native::(anonymous namespace)::_vec_log_softmax_lastdim<float>(float*, float*, long, long)::'lambda'(long, long)::operator()(long, long) const::'lambda'(at::vec256::(anonymous namespace)::Vec256<float>&, at::vec256::(anonymous namespace)::Vec256<float>&)>(void at::native::(anonymous namespace)::_vec_log_softmax_lastdim<float>(float*, float*, long, long)::'lambda'(long, long)::operator()(long, long) const::'lambda'(at::vec256::(anonymous namespace)::Vec256<float>&, at::vec256::(anonymous namespace)::Vec256<float>&) const&, at::vec256::(anonymous namespace)::Vec256<float>, long) xplat/caffe2/aten/src/ATen/cpu/vec256/functional.h:12
```
Test Plan:
passed sanitizer locally after change,
CI green
Differential Revision: D21408120
fbshipit-source-id: b9d058cedf42b3d1d34ce05a42049d402906cd13
