Fix hunyuan video attention mask dim (huggingface#10454)

* fix

* add coauthor

Co-Authored-By: Nerogar <[email protected]>

---------

Co-authored-by: Nerogar <[email protected]>

Author: a-r-r-o-w

src/diffusers/models/transformers/transformer_hunyuan_video.py

@@ -721,6 +721,7 @@ def forward(
 
         for i in range(batch_size):
             attention_mask[i, : effective_sequence_length[i], : effective_sequence_length[i]] = True
+        attention_mask = attention_mask.unsqueeze(1)  # [B, 1, N, N], for broadcasting across attention heads
 
         # 4. Transformer blocks
         if torch.is_grad_enabled() and self.gradient_checkpointing: