[Submodule] Change FlashInfer to import

[comaniac]

This PR removes submodule flashinfer from this repo. As flashinfer is now officially released, we can directly install it via pip. However, I didn't add it to pyproject.toml but just updated the README due to two reasons:

1. flashinfer is not available on PYPI yet.
2. It supports limited CUDA version and requires to manually select the wheel path.

Meanwhile, the latest version of flashinfer has some interface changes. I modified them according to this comparison:
flashinfer-ai/flashinfer@88b9496...3fc62cb

I verified two things with this PR:

1. My test prompt gets the same output tokens before and after this PR.
2. benchmark/multi_chain_reasoning generates different results even temperature=0. There might be due to the misuse of the new interface or the randomness in the kernel. I'll do more investigation.

cc @Ying1123

[comaniac]

Per offline discussion, I instead used gsm8k with 20 questions. The outputs with and without this PR are 99% the same, so we should be good to go.