[Tokenizers] Port CodeGenTokenizer & byte-level BPE algorithm

[ericstj]

Port Codegen Tokenizer to enable Phi-2 models

Reference:
https://huggingface.co/docs/transformers/main/en/model_doc/codegen
https://github.com/huggingface/transformers/blob/0549000c5bf6c7249f411917f2a6f0b6d0f06da1/src/transformers/models/codegen/tokenization_codegen.py#L98

Paper:
https://arxiv.org/abs/2203.13474
https://arxiv.org/pdf/2203.13474.pdf

[tarekgh]

This is already done!