[torch] Remove using directive from header (#59728)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59728
I noticed Sandcastle jobs failing with:
```
fbcode/caffe2/torch/csrc/api/include/torch/nn/modules/rnn.h:19:35: error: using namespace directive in global context in header [-Werror,-Wheader-hygiene]
using namespace torch::nn::utils::rnn;
```
(cf. V3 of D28939167 or https://www.internalfb.com/intern/sandcastle/job/36028797455955174/).
Removing `using namespace ...` fixes the problem.
~~... also applied code formatting ...~~
Test Plan: Sandcastle
Reviewed By: jbschlosser
Differential Revision: D29000888
fbshipit-source-id: 10917426828fc0c82b982da435ce891dc2bb6eec
