[Typing] Mypy typing part 2

[rkooo567]

NOTE: There are many fields that are lazy initialized and assume these are accessed only after lazy initialization is done. I fixed them by using the solution suggested in this approach; https://stackoverflow.com/questions/60925137/using-mypy-with-with-lazy-initialization-of-instance-attributes

Handles some parts of #3680

mypy vllm/engine/*.py --follow-imports=skip --config-file pyproject.toml
mypy vllm/worker/*.py --follow-imports=skip --config-file pyproject.toml
mypy vllm/spec_decode/*.py --follow-imports=skip --config-file pyproject.toml
mypy vllm/model_executor/*.py --follow-imports=skip --config-file pyproject.toml

Remaining:

# mypy vllm/lora/*.py --follow-imports=skip --config-file pyproject.toml

And some other nested directories (such as model_executors/layer)

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[rkooo567]

all these engine related code seems a bit hacky and it was difficult to fix

[rkooo567]

@sangstar I found this approach is a bit hacky (dynamically loading load_format) and doesn't work well with typing. Is there any good suggestion to fix it?

[sangstar]

I believe @Yard1 's refactor addressed this :D

[rkooo567]

cc @simon-mo @zhuohan123 this PR is ready to review

[simon-mo]

Thanks for working on this. I made a quick skim.

[simon-mo]

not sure why is this added back. this might be from a bad merge..

[rkooo567]

yep bad merge. fixed!

[rkooo567]

@simon-mo I think we can merge it (I believe spec decoding failure is from the master)