[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.
<!--
copilot:poem
-->
### <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
diff --git a/test/test_mps.py b/test/test_mps.py
index c844444..7a06efc 100644
--- a/test/test_mps.py
+++ b/test/test_mps.py
@@ -3565,6 +3565,8 @@
helper((1, 5), (4, 0, 5), src_dtype, dst_dtype)
helper((3, 1, 0), (3, 5, 0), src_dtype, dst_dtype)
helper((0, 1, 0), (0, 5, 0), src_dtype, dst_dtype)
+ # Regression test for https://github.com/pytorch/pytorch/issues/107867
+ self.assertEqual(torch.tensor([[1]], device='mps').item(), 1.0)
# See https://github.com/pytorch/pytorch/pull/84742
# and https://github.com/pytorch/pytorch/pull/78319
