[Core] Support all head sizes up to 256 with FlashAttention backend

[njhill]

We were previously restricting to specific sizes, but the native FA kernels pad and support arbitrary sizes up to 256.

[tlrmchlsmth]

Could you add some unit tests? Looks like we may be able to just extend this list here🤞

vllm/tests/kernels/test_attention.py

Lines 32 to 34 in c2ec430

	# FlashAttention forward only supports head dimension at most 128
	# https://github.com/ROCmSoftwarePlatform/flash-attention/blob/3d2b6f5d037782cc2c906909a46fb7e2e1b48b25/csrc/flash_attn_rocm/flash_api.cpp#L62
	HEAD_SIZES = [64, 80, 96, 112, 120, 128, 192, 256]

[njhill]

Looks like we need to build flash without the FLASHATTENTION_DISABLE_UNEVEN_K flag, have opened vllm-project/flash-attention#21 ... @WoosukKwon wdyt?