Griffin-Lim phase init (issue 3828)

[michiel-j]

Proposed solution to issue 3828 "Ability to provide initial phase to Griffin-Lim" (link to issue 3828).
Change: if provided spectrogram is complex (and rand_init=False), then its phase is used as initialisation for Griffin-Lim phase estimation algorithm.
I also changed the docs of both src/torchaudio/transforms/_transforms.py and src/torchaudio/functional/functional.py.

Code used to test proposed solution (messy code): https://gist.github.com/michiel-j/740f704e50a4f812273fa9c5f88ae982

[pytorch-bot]

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