[Misc] refactor ops and cache_ops layer

[jikunshang]

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Refactor ops and cache_ops layer. Add an abstraction ops layer and will use vllm._C.ops by default.
This would be easier to add/extend other third party high performance ops/kernels implementation if necessary.

FIX #xxxx (link existing issues this PR will resolve)

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[WoosukKwon]

@jikunshang Thanks for submitting the PR! Please check out my comments.

[WoosukKwon]

Do we really need this class? Why don't we just directly define the functions below and use from vllm import ops instead of from vllm.ops import ops? (we don't need to keep cache_ops)

[jikunshang]

remove class ops and cache_ops. since we renamed the file, import be like: from vllm import _custom_ops as ops

[WoosukKwon]

Here, we are making the assumption that the function signatures of the custom ops are the same across different backends. While we already rely on this assumption, I feel this may not be necessarily true in the future.

That being said, I feel adding this layer of indirection doesn't hurt us at the moment so it's a good addition to vLLM.

[WoosukKwon]

BTW, why don't we rename this file to _custom_ops.py or something like that?

[jikunshang]

Yes, for further backends, they need to align with custom ops function signature, or try to adapt it in their ops layer.

Renamed the file.

[WoosukKwon]

@jikunshang LGTM. Thanks!