process_encoder_hidden_states function is applied on each denoising step

https://github.com/huggingface/diffusers/blob/31058cdaef63ca660a1a045281d156239fba8192/src/diffusers/models/unets/unet_2d_condition.py#L1004

Hey! I noticed that `process_encoder_hidden_states` is applied on each denoising step that influences on an inference performance. This function can be executed only once before running UNet2DConditionModel.forward. The output of `process_encoder_hidden_states` should be combined with `encoder_hidden_states` and be provided to UNet2DConditionModel.forward