Modify installation instructions

[samanklesaria]

This PR updates the installation instructions for each platform to remove references to ffmpeg.

[pytorch-bot]

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[pearu]

This PR removes FFmpeg related installation docs. There are still many references to ffmpeg in src and elsewhere (see grep -r -I --exclude "*~" -i ffmpeg .). I think it would be clearer if all references/usages of ffmpeg are removed using a single PR.

Same note applies to other deleted features in torchaudio.

[samanklesaria]

As far as occurances of 'ffmpeg' remaining, there's a couple in .github/workflows, there's some lines in CMakelists, there's a smoke test (that I remember Nicholas advocating we keep for backward compatability), there's the torchcodec loading tests (which will be kept), and there's some flags for including parts of common_utils. I was planning on keeping this PR just about installation instructions. How about I make a second one for the CMakelist removals, common_utils flag removals, and github workflow removals?

[samanklesaria]

Most of the occurances of 'sox' you're seeing are for sox compatibility tests, which we want to preserve. What remains is:

• A bunch of references to http://sox.sourceforge.net/sox.html in filtering.py which are just saying that the torchaudio function replications the functionality of a sox function.
• The apply_codec function, which is deprecated and needs to be removed. I'll address this function in a separate PR.

[pearu]

LGTM! Thanks, @samanklesaria!