Support out_dim argument for Attention block

**Is your feature request related to a problem? Please describe.**
When i feed the `out_dim` argument in `__init__` in [Attention block](https://github.com/huggingface/diffusers/blob/b69fd990ad8026f21893499ab396d969b62bb8cc/src/diffusers/models/attention_processor.py#L114) it will raise the shape error, because the `query_dim != out_dim`. In this case, the following code try to keep the given channel of `hidden_states`.
> https://github.com/huggingface/diffusers/blob/b69fd990ad8026f21893499ab396d969b62bb8cc/src/diffusers/models/attention_processor.py#L1393
But it should change the channel as the output of `hidden_states = attn.to_out[0](hidden_states)`.

**Describe the solution you'd like.**
I suggest the change of code base : https://github.com/huggingface/diffusers/blob/b69fd990ad8026f21893499ab396d969b62bb8cc/src/diffusers/models/attention_processor.py#L1393
to `hidden_states = hidden_states.transpose(-1, -2).reshape(batch_size, -1, height, width)`, then it will respect the channel of `hidden_states`.
Maybe I will make a PR later.

**Describe alternatives you've considered.**
None.

**Additional context.**
None.