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This fixes flash attention v1 which was always
NotImplementedError("window_size_left is only available with flash attn
v2").

Currently flash_llama_modeling.py doesn't override the default value of
window_size_left when calling attention(..) (line 282). This means that
window_size_left will always be the default of -1, but flash attention
v1 throws an exception if `window_size_left != 0`.

To fix this, we should be checking `window_size_left != -1` before
throwing the NotImplementedError.


Fixes #1084 


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@OlivierDehaene OR @Narsil