Switch to PyTorch Tokenizers

[IgorSwat]

ExecuTorch 1.0 comes with its own tokenizers implementation based on PyTorch Tokenizers. Since it is already being bundled into ExecuTorch binaries, we could get rid of tokenizers-cpp package and reduce the bundle size of our library.

[msluszniak]

This one is really important and I will escalate this with important label since I saw a few comments on reddit, e.g. https://www.reddit.com/r/reactnative/comments/1ooxm5c/comment/nndrpi3/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button that bundles created after adding RNE are too big for just one small feature in the app.