[AutoModel] fix `torch_dtype=auto` in `from_pretrained`
This PR:

1. fixes the case of `torch_dtype=auto` in `AutoModel.from_pretrained` which got unintentionally stripped in https://github.com/huggingface/transformers/pull/21524 - now `torch_dtype=auto` gets always passed on to the  `from_pretrained` method of the resolved class.
2. adds a test

Fixes: https://github.com/huggingface/transformers/issues/23357
_The documentation is not available anymore as the PR was closed or merged._<|||||>The only change is that `torch_dtype="auto"` remains in `kwargs` and it wasn't before.

I have just flipped around the `_copy` vs `_orig` as it looked simpler to read that way. There is no functional change in that part of the code.

Probably could just set a flag of `is_torch_dtype_auto = True` instead of copying `kwargs` - I just thought that perhaps down the road other entries might need a special handling. Let me know if you prefer that I recode to use the flag instead. It surely would be cleaner I think.<|||||>No no, that works as is.