[Bugfix] add hf_token to EngineArgs

[paolovic]

Added hf_token to EngineArgs. It stores True if just the flag and no value is given, it stores the value if the flag and value are given, and evaluates to None otherwise.
Also, it can be passed to LLM as an argument where it's forwarded in the __init__ to EngineArgs where it's stored as an attribute and forwarded to ModelConfig in create_model_config.
Within ModelConfig it's available to be passed to get_hf_image_processor_config which forwards it to transformers' get_image_processor_config.

FIX 14854

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[paolovic]

branched out a new source branch such that I am not blocked until @void-mckenzie gives his feedback

[WoosukKwon]

QQ: Why not just use env variables like HF_TOKEN or HF_HOME?

[paolovic]

Hi @WoosukKwon ,
void-mckenzie requested this feature like the following:
"It works fine if you just have one HF token. But if you have multiple, you need to reset the environment variable every time. Wouldn't it make more sense to use the inbuilt auth token functionality in transformers? It exists to support multiple token access. I have an issue right now with my unsloth build, since vllm cuts off the auth token pathway from unsloth to transformers."

In the following DarkLight1337 and him agreed on the design that is implemented with this PR.
#14854

[DarkLight1337]

Since the OP of the original issue hasn't responded, I'll just merge this first

[void-mckenzie]

@paolovic Sorry! been busy with a few things. I see that this has been merged, let me try to see if I can pass the tokens. I'll keep you updated. once again, sorry for the delay!