Fixed STFT.inverse() could not work correct under batch operations

[mcw519]

Fix for #100

[KinWaiCheuk]

It seems GitHub Action is not running a unit test on your repo.
https://github.com/mcw519/nnAudio/actions

Usually when you push a commit to your master branch, GitHub will automatically run a unit test like this
https://github.com/THasthika/nnAudio/actions

If it is the case, I might need to run the unit test manually first.

[mcw519]

Oh, I feel sorry about that. Maybe cause I directly made the commit on GitHub page so not trigger the unit test?

[KinWaiCheuk]

I am not sure, but it's okay, I have run the unit test locally. Everything looks good to me.