Migrate to PyTorch tokenizers

[benITo47]

Description

This PR migrates us from tokenisers-cpp to PyTorch tokenisers bundled with executorch

Introduces a breaking change?

• Yes
• No - User faces no changes

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

This changes need to be tested manually. Try running all our apps that consume tokenizers and see whether the output is ok.
By "OK" I mean

• no unwanted special tokens
• response that is related to the question (if model start answering about aquariums after being asked about ketchup, then it's not ok)
• Output has proper punctuation, no missing or added spaces.

Related issues

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings