Add tf.spectral.dct, based on scipy.fftpack.dct.
Add tf.spectral.dct, based on scipy.fftpack.dct.

Only supports the type II DCT for the moment, but implements SciPy's API to fully match it once type I and III are implemented.

Implemented using a length 2N RFFT, as described here:
https://dsp.stackexchange.com/a/10606

PiperOrigin-RevId: 170808354

tensorflow/docs,tensorflow/docs-l10n,tensorflow/docs,tensorflow/docs