[tfjs-core] Performance issue with tf.signal.stft (#4623) (#4790)
[tfjs-core] Performance issue with tf.signal.stft (#4623) (#4790)

BUG
* stft_ remove unneccesary split, concat

* removed unneccesary imports

* typo correction - tests run without errors

Co-authored-by: Rajeshwar Reddy T <81ddf3010ddb2346b6246603d3a55c487a2e7769@users.noreply.github.com>
Co-authored-by: Ping Yu <e3982a96662cc73e971d68f6aafe25f9d2e93e85@users.noreply.github.com>
tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs