Update how we input kwargs (#314)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/314

Previously, the code for passing inputs to exported program was:
```
if kwargs:
    return (args, kwargs)
else:
    return args
```

However, this causes some inconsistency where if the original input contains args and kwargs, the treespec would be a tuple containing a tuple of arguments, and a dictionary of keyword arguments. But if the original input only contained args, the treespec would just be a tuple of arguments.

So I updated the code to just always keep the kwargs around

cc voznesenskym penguinwu EikanWang jgong5 Guobing-Chen XiaobingSuper zhuhaozhe blzheng Xia-Weiwen wenzhe-nrv jiayisunx chenyang78 aakhundov kadeng

X-link: https://github.com/pytorch/pytorch/pull/109160

Reviewed By: zhxchen17

Differential Revision: D49218534

Pulled By: angelayi

fbshipit-source-id: dbc481aaa93df349dc3171a1d66a306ec4dd2370