[I2VGenXL] `attention_head_dim` in the UNet (#6872)
* attention_head_dim
* debug
* print more info
* correct num_attention_heads behaviour
* down_block_num_attention_heads -> num_attention_heads.
* correct the image link in doc.
* add: deprecation for num_attention_head
* fix: test argument to use attention_head_dim
* more fixes.
* quality
* address comments.
* remove depcrecation.
