Refactor jestream to allow different tokenizers

[qihqi]

Issue

Currently we assume few things in jetstream which hinders it's generalization:

1. tokenizer is SentencePiece based.
2. pad_id is 0
3. after encode, we pad to nearest power of 2
4. ResultToken itself is jax specific (the @struct.dataclass annotation requires it's Jax pytreeable).

These assumptions hinders generalization (i.e. support wider varieties of models).
Examples:

1. llama3 uses tiktoken instead of SentencePiece
2. llama3 uses pad_id of -1
3. Pytorch GPU does NOT need to pad to nearst power of 2.
4. Pytorch GPU version of jetstream would like to use torch.Tensor to hold the data, which is not jax-pytreeable.

Proposal:

1. EngineAPI.get_tokenizer which returns the tokenizer, should be any object that implements the following interface:

def encode

def decode

@property
def pad_id

@property
def eos_id

Uses of tokenizer should restrict to only this methods.

In particular: encode should do both encoding and padding. So jetstream doesnt do any padding itself; the engine can choose how to pad (or not to pad) by returning a custom tokenizer object whose encode also does the padding.

2. Allow use different implementation for ResultTokens; same as Prefix and DecodeState. Implementations of the Engine can choose implementation of ResultTokens. jestream should interact with it only through it's 3 public methods (https://github.com/google/JetStream/blob/main/jetstream/engine/engine_api.py#L83)

def get_result_at_slot
def convert_to_numpy
def copy_to_host_async

and are not allowed to access it's fields directly.

[bhavya01]

#53, #67 adds the interface for tokenizer. Still need to work on ResultTokens.