Initialize flash attn flag (#38768)
_flash_supports_window_size is used further down in this file and relied on by e.g. [ring-flash-attention](https://github.com/zhuzilin/ring-flash-attention/blob/123f924/ring_flash_attn/adapters/hf_adapter.py#L9-L11). Even though it is an unexported name, it still makes sense to keep the state of `globals()` in this file consistent.
