Update SAM AMG README with more descriptive install instructions

[philipbutler]

@cpuhrsch
Installing was not so obvious, so this should help others trying to test and contribute.

There's some extra commits from amending, I'm still kinda garbage at git

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/1337

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[cpuhrsch]

The download part I guess doesn't apply anymore now that the user can get this via the model type argument.

[cpuhrsch]

Looks good, let me know once you got a chance to give this another commit or two and then we can merge.

[cpuhrsch]

Ah, these are the sam2 configs. There's sam2 and then there's sam2.1. I had to switched to sam2.1, but maybe sam2 is still useful? I updated the sam2.1 configs in b2e42ff#diff-12caf984b30e7028df7fb867a984c5ccad3fde0a0783df2af26db9cef860fbe5 , but in any case it doesn't hurt to have sam2 support as well.