use base version for version comparison

[BoyuanFeng]

#18846 turns standalone_compile on by default via is_torch_equal_or_newer("2.8.0"). One major motivation is to use standalone_compile for vllm x torch nightly CI tests.

However, it has no effect yet. The current torch nightly gives 2.8.0a0+... which is a pre-release and is considered to be less than 2.8.0. This PR fixes the issue by comparing with 2.8.0a.

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[zou3519]

cc @drisspg @jerryzh168, turns out standalone_compile was not enabled by default for nightlies yet. The flag VLLM_USE_STANDALONE_COMPILE=1 still works for testing.

[houseroad]

Sorry, what does "a" in "2.8.0a" mean here?

[BoyuanFeng]

'a' is a standard version for alpha: https://peps.python.org/pep-0440/
current pytorch nightly has the version 2.8.0a0+git093fd47

[houseroad]

I see, thanks for the explanation. So the version parser thinks 2.8.0 > 2.8.0a then.