Support flash-attn 2.7.0 in FPDT attention (#8022)
## Summary
- flash-attn 2.7.0 split the `window_size` argument into
`window_size_left`/`window_size_right` ints, and reduced
`_flash_attn_forward`'s return from 8 values to 4. Calling FPDT with
flash-attn >= 2.7 currently errors out.
- Add a `>= 2.7.0` branch to both `_FPDTGPUAttentionImpl_` and
`_FPDTGPUOffloadingAttentionImpl_` (forward + backward) so FPDT keeps
working with newer flash-attn releases while remaining compatible with
2.6.x and earlier.
- Update `FPDT_Attention`'s ImportError message so it no longer pins
2.6.3.
Signed-off-by: binchengxiong <binchengxiong@alibaba-inc.com>
Co-authored-by: binchengxiong <binchengxiong@alibaba-inc.com>
Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: Masahiro Tanaka <81312776+tohtana@users.noreply.github.com>
