remove torch dependency

[msaroufim]

When users are running

pip install torchao

This is causing a force downgrade to torch==2.3.1

Instead we should tell users that if they want CPP extensions working with an official release they should install torch==2.3.1 in documentation but we should never force downgrade

So with this PR we remove the pytorch dependency completely and instead rely on building torchao without build isolation to make sure torch is available as a build time dependency

This is an unfortunate workaround because pyproject.toml is not expressive enough to pick a dynamic version for torch

Todo: figure out the right test strategy for this PR

[pytorch-bot]

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[malfet]

IMO you need to keep runtime requirement on torch, as torchao indeed needs one
And if you want to warn users about the pytorch version it was build against, you can do it in __init__.py or something like that