PixArtTransformer2DModel  
A 2D Transformer model as introduced in PixArt family of models (https://arxiv.org/abs/2310.00426,
https://arxiv.org/abs/2403.04692).  
Parameters:
num_attention_heads (int, optional, defaults to 16): The number of heads to use for multi-head attention.
attention_head_dim (int, optional, defaults to 72): The number of channels in each head.
in_channels (int, defaults to 4): The number of channels in the input.
out_channels (int, optional):