unet.py file miss count_flops_attn() function.

[VoldemortGin]

In guided_diffusion.unet.py file, the method count_flops() of 2 classes (QKVAttentionLegacy class and QKVAttention class) need a function with signature count_flops_attn(model, _x, y). The method is not been called, so there is no error, but I think it's better to adding it.