[Bugfix] Massage MLA's usage of flash attn for RoCM

[tlrmchlsmth]

This PR massages some vllm_flash_attn vs flash_attn interface differences that appeared between a couple of PRs:

#12662 added a this fallback to flash_attn in order to support MLA on RoCM:

vllm/vllm/attention/backends/mla/utils.py

Lines 33 to 36 in 5e5c8e0

	try:
	from vllm.vllm_flash_attn import flash_attn_varlen_func
	except ImportError:
	from flash_attn import flash_attn_varlen_func

Subsequently #12807 updated to use the vllm_flash_attn specific arguments that control whether we use FA2 or FA3.

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