Support flash-attn 2.7.0 in FPDT attention

[xbcReal]

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.

[xbcReal]

If we don't add the uploaded commit, we would come across the following error message when running the test command using flash-attn 2.7.0 version:
pytest -v sequence_parallelism/test_ulysses.py::TestFPDTAttention::test_FPDT_attention_offloading_output_consistency[32-8-128-2048-4]

RuntimeError: flash_attn::_flash_attn_forward() is missing value for argument 'window_size_left'. Declaration: flash_attn::_flash_attn_forward(Tensor q, Tensor k, Tensor v, float dropout_p, float softmax_scale, bool causal, SymInt window_size_left, SymInt window_size_right, float softcap, Tensor? alibi_slopes, bool return_softmax) -> (Tensor, Tensor, Tensor, Tensor)

[tohtana]

@YJHMITWEB Can you review this PR?

[YJHMITWEB]

@YJHMITWEB Can you review this PR?

Hi @tohtana , this PR looks good to me, as it only includes a version check. We can bring it in with a rebase.

[tohtana]

Thank you @xbcReal! Let me merge this.

@YJHMITWEB Thank you for your review!