[MPS] Fix `.item()` for multi-dim scalar (#107913)
By refactoring `_local_scalar_dense_mps` to use `_empty_like` to allocate CPU tensor.
Also, print a more reasonable error message when dst dim is less than src in mps_copy_
This fixes regression introduced by https://github.com/pytorch/pytorch/pull/105617 and adds regression test.
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### <samp>🤖 Generated by Copilot at abd06e6</samp>
> _Sing, O Muse, of the valiant deeds of the PyTorch developers_
> _Who strive to improve the performance and usability of tensors_
> _And who, with skill and wisdom, fixed a bug in the MPS backend_
> _That caused confusion and dismay to many a user of `item()`_
Fixes https://github.com/pytorch/pytorch/issues/107867
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107913
Approved by: https://github.com/albanD
