Update attention.py by adding SparseCausalAttention implementation

[vancyland]

Add SparseCausalAttention implementation to ./animatediff/models/attention.py

If you want to use SparseCausalAttention in the self-attention part, simply set unet_use_cross_frame_attention = True.

If you want to further improve the code, you can focus on the class SparseCausalAttention2D(CrossAttention).

To view the corresponding experimental results, please visit this link.