invoke NN smoketests from a python loop instead of a batch file (#18756)
Summary:
I tried first to convert the `.bat` script to a Bash `.sh` script, but I got this error:
```
[...]/build/win_tmp/ci_scripts/test_python_nn.sh: line 3: fg: no job control
```
Line 3 was where `%TMP_DIR%/ci_scripts/setup_pytorch_env.bat` was invoked.
I found a potential workaround on stack overflow of adding the `monitor` (`-m`) flag to the script, but hat didn't work either:
```
00:58:00 /bin/bash: cannot set terminal process group (3568): Inappropriate ioctl for device
00:58:00 /bin/bash: no job control in this shell
00:58:00 + %TMP_DIR%/ci_scripts/setup_pytorch_env.bat
00:58:00 /c/Jenkins/workspace/pytorch-builds/pytorch-win-ws2016-cuda9-cudnn7-py3-test1/build/win_tmp/ci_scripts/test_python_nn.sh: line 3: fg: no job control
```
So instead I decided to use Python to replace the `.bat` script. I believe this is an improvement in that it's both "table-driven" now and cross-platform.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18756
Differential Revision: D14957570
Pulled By: kostmo
fbshipit-source-id: 87794e64b56ffacbde4fd44938045f9f68f7bc2a
