[Kernel] Revert the API change of Attention.forward

[heheda12345]

The changing of kv_cache -> _kv_cache and attn_metadata -> _attn_metadata in Attention.forward by #11967 breaks models that pass these two arguments with kwargs, e.g., phi3_small:

attn_output = self.attn(q, k, v, kv_cache, attn_metadata=attn_metadata)

And this script is crashed

from vllm import LLM, SamplingParams

# Sample prompts.
prompts = [
    "Hello, my name is",
    "The president of the United States is",
    "The capital of France is",
    "The future of AI is",
]
# Create a sampling params object.
sampling_params = SamplingParams(temperature=0.8, top_p=0.95)

# Create an LLM.
llm = LLM(model="microsoft/Phi-3-small-8k-instruct", trust_remote_code=True)
# Generate texts from the prompts. The output is a list of RequestOutput objects
# that contain the prompt, generated text, and other information.
outputs = llm.generate(prompts, sampling_params)
# Print the outputs.
for output in outputs:
    prompt = output.prompt
    generated_text = output.outputs[0].text
    print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")

This pr reverts the API change to fix this problem.
CC @youkaichao
NOTE: the above script is still crashed due to other problems. I'm investigating and fixing it.

[github-actions]

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[youkaichao]

LGTM, thanks for the fix!

[heheda12345]

Fixing phi3small model by #12040