Fix Flash Attention 3 backward API parameter naming

Rename causal parameter to is_causal in flash_attn_bwd function to align
with flash-attn v2.7.0+ API changes. This ensures consistency with the
updated flash-attn library interface for backward pass operations.

Signed-off-by: meichaoyang001 <meichaoyang001@ke.com>

Author: jhvmhg

transformer_engine/pytorch/attention/dot_product_attention/context_parallel.py

@@ -1225,6 +1225,10 @@ def cp_p2p_bwd_flash_attn(
         dk=dk,
         dv=dv,
     )
+    if use_flash_attn_3:
+        fa_backward_kwargs["is_causal"] = causal_
+    else:
+        fa_backward_kwargs["causal"] = causal_
     flash_attn_bwd(
         dout_part,
         q_part,
@@ -3213,6 +3217,10 @@ def backward(ctx, dout, *_args):
                         elif ctx.use_flash_attn_3 or fa_utils.v2_7_0_plus:
                             fa_backward_kwargs["window_size_left"] = window_size_per_step[i][0]
                             fa_backward_kwargs["window_size_right"] = window_size_per_step[i][1]
+                        if ctx.use_flash_attn_3:
+                            fa_backward_kwargs["is_causal"] = "causal" in ctx.attn_mask_type
+                        else:
+                            fa_backward_kwargs["causal"] = "causal" in ctx.attn_mask_type
                         flash_attn_bwd(
                             dout_,
                             q_,
@@ -3221,7 +3229,6 @@ def backward(ctx, dout, *_args):
                             out_,
                             softmax_lse_per_step[i],
                             *fa_backward_args_thd,
-                            causal="causal" in ctx.attn_mask_type,
                             **fa_backward_kwargs,
                         )
 
@@ -3821,8 +3828,11 @@ def backward(ctx, dout, *_args):
                 dk=dk,
                 dv=dv,
             )
-            if not ctx.use_flash_attn_3:
+            if ctx.use_flash_attn_3:
+                fa_backward_kwargs["is_causal"] = causal
+            else:
                 fa_backward_kwargs["rng_state"] = rng_state
+                fa_backward_kwargs["causal"] = causal
             flash_attn_bwd(
                 dout,
                 q,
@@ -3831,7 +3841,6 @@ def backward(ctx, dout, *_args):
                 out,
                 softmax_lse,
                 *fa_backward_args_thd,
-                causal=causal,
                 **fa_backward_kwargs,
             )