initial effort of adding fa4

Signed-off-by: Xin Yao <xiny@nvidia.com>

Author: yaox12

transformer_engine/pytorch/attention/dot_product_attention/backends.py

@@ -80,17 +80,17 @@
 from transformer_engine.pytorch.export import is_in_onnx_export_mode
 from transformer_engine.pytorch.graph import is_graph_capturing
 
-# Global vars for flash attn v2 and v3 imports
-flash_attn_cuda_bwd = None
-flash_attn_func = None
-flash_attn_varlen_func = None
-_flash_attn_fwd = None
-_flash_attn_bwd = None
-_flash_attn_varlen_fwd = None
-_flash_attn_varlen_bwd = None
+# Try to import Flash Attention v2
 try:
     fa_utils.version = PkgVersion(get_pkg_version("flash-attn"))
 except PackageNotFoundError:
+    flash_attn_cuda_bwd = None
+    flash_attn_func = None
+    flash_attn_varlen_func = None
+    _flash_attn_fwd = None
+    _flash_attn_bwd = None
+    _flash_attn_varlen_fwd = None
+    _flash_attn_varlen_bwd = None
     pass  # only print warning if use_flash_attention_2 = True in get_attention_backend
 else:
     if torch.cuda.is_available() and get_device_compute_capability() >= (10, 0):
@@ -130,12 +130,16 @@
             ),
             fa_utils.version,
         )
+
+# Try to import Flash Attention v3
 try:
     fa_utils.fa3_version = PkgVersion(get_pkg_version("flash-attn-3"))
 except PackageNotFoundError:
     flash_attn_func_v3 = None
     flash_attn_varlen_func_v3 = None
     flash_attn_with_kvcache_v3 = None
+    _flash_attn_fwd_v3 = None
+    _flash_attn_bwd_v3 = None
     # pass  # only print warning if use_flash_attention_3 = True in get_attention_backend
 else:
     from flash_attn_3.flash_attn_interface import flash_attn_func as flash_attn_func_v3
@@ -150,6 +154,24 @@
 
     fa_utils.set_flash_attention_3_params()
 
+# Try to import Flash Attention v4
+try:
+    fa_utils.fa4_version = PkgVersion(get_pkg_version("flash-attn-cute"))
+except PackageNotFoundError:
+    flash_attn_func_v4 = None
+    flash_attn_varlen_func_v4 = None
+    flash_attn_with_kvcache_v4 = None
+    _flash_attn_fwd_v4 = None
+    _flash_attn_bwd_v4 = None
+    # pass  # only print warning if use_flash_attention_4 = True in get_attention_backend
+else:
+    from flash_attn.cute.interface import flash_attn_func as flash_attn_func_v4
+    from flash_attn.cute.interface import flash_attn_varlen_func as flash_attn_varlen_func_v4
+    from flash_attn.cute.interface import _flash_attn_fwd as _flash_attn_fwd_v4
+    from flash_attn.cute.interface import _flash_attn_bwd as _flash_attn_bwd_v4
+    # flash_attn_with_kvcache_v4 = None  # FA4 does not support kvcache yet
+    fa_utils.set_flash_attention_4_params()
+
 # Float8CurrentScaling: fused_attn_bwd takes O in FP8 by default, this flag allows it in F16
 _dpa_fp8_cs_o_in_f16 = os.getenv("NVTE_DPA_FP8CS_O_in_F16", "1") == "1"
 
@@ -859,6 +881,9 @@ def forward(
         use_flash_attn_3 = False
         if flash_attention_backend is not None and flash_attention_backend > PkgVersion("3.0.0b"):
             use_flash_attn_3 = True
+        use_flash_attn_4 = False
+        if flash_attention_backend is not None and str(flash_attention_backend).endswith("cute"):
+            use_flash_attn_4 = True
         if context_parallel and all(
             not isinstance(x, Float8Tensor) for x in [query_layer, key_layer, value_layer]
         ):
@@ -913,9 +938,13 @@ def forward(
                 #       |                         |     bshd/sbhd/thd + padding
                 fa_optional_forward_args_thd = []
                 if qkv_format in ["bshd", "sbhd"] and "padding" not in attn_mask_type:
-                    func = (
-                        flash_attn_func if not use_flash_attn_3 else flash_attn_func_v3
-                    )  # pylint: disable=possibly-used-before-assignment
+                    func = None
+                    if use_flash_attn_4:
+                        func = flash_attn_func_v4
+                    elif use_flash_attn_3:
+                        func = flash_attn_func_v3
+                    else:
+                        func = flash_attn_func
                 else:
                     if not use_flash_attn_3:
                         func = flash_attn_varlen_func
@@ -928,7 +957,24 @@ def forward(
                         fa_optional_forward_args_thd.append(cu_seqlens_kv)
                         fa_optional_forward_args_thd.append(max_seqlen_q)
                         fa_optional_forward_args_thd.append(max_seqlen_kv)
-                if not use_flash_attn_3:
+                if use_flash_attn_4:
+                    fa_4_optional_forward_kwargs = {
+                        # "window_size": window_size,
+                        "num_splits": num_splits,
+                    }
+                    if inference_params is None:
+                        fa_4_optional_forward_kwargs["deterministic"] = self.deterministic
+                    output = func(
+                        query_layer,
+                        key_layer,
+                        value_layer,
+                        softmax_scale=self.softmax_scale,
+                        causal="causal" in attn_mask_type,
+                        **fa_4_optional_forward_kwargs,
+                    )
+                    if isinstance(output, (List, Tuple)):
+                        output = output[0]
+                elif not use_flash_attn_3:
                     fa_optional_forward_kwargs = {}
                     if fa_utils.v2_3_plus:
                         fa_optional_forward_kwargs["window_size"] = window_size