[from_pretrained] extend `torch_dtype="auto"` to look up `config.torch_dtype` first, expand docs
1. As discussed at https://huggingface.slack.com/archives/C01N44FJDHT/p1675880091973589 this PR proposes to expand the `from_pretrained` documentation of how `torch_dtype`'s different values impact the model loading.
2. next `torch_dtype="auto"` was expanded to first look up `config.torch_dtype` and if it's not there try to derive it from weights.
3. fixed a bug that was polluting the config object when `torch_dtype="auto"`
4. Tests were expanded to test the new functionality and the bug.


_The documentation is not available anymore as the PR was closed or merged._