[Bugfix] add input embedding

[Bryce1010]

adds support for passing prompt_embeds to LLM.generate as

llm.generate({"prompt_embeds": input_embeds}, sampling_params)

or

llm.generate(
    [{"prompt_embeds": input_embeds} for input_embeds in inputs_embeds], sampling_params
)

this enables use cases when only the embedding layer is finetuned, and have the same model backend support multiple custom tuned embedding layers

FIX #416
FIX #8323

[github-actions]

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[DarkLight1337]

Thanks for opening this PR, can you explain what this PR is about and how this is related to #11375?

[Bryce1010]

Thanks for opening this PR, can you explain what this PR is about and how this is related to #11375?

@DarkLight1337 Sorry for referencing the incorrect issue number. Please refer to the following issues for
#416
#8323

[DarkLight1337]

There is a related PR #6869, but your solution can sidestep the problem of missing input token IDs (at the cost of being less efficient).

Can you update this PR with the unit tests in #6869 to see whether this solution works correctly?