Fix flaky TestVmap.test_vmap_masked_scatter

[zcbenz]

This test is flaky in CI and luckily I can reproduce it locally:

  ======================================================================
  FAIL: test_vmap_masked_scatter (test_vmap.TestVmap.test_vmap_masked_scatter)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "/home/runner/work/mlx/mlx/python/tests/test_vmap.py", line 900, in test_vmap_masked_scatter
      self.assertTrue(mx.array_equal(expected, out))
      ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  AssertionError: array(False, dtype=bool) is not true
  
  ----------------------------------------------------------------------

[angeloskath]

Hmm the previous code is actually correct in the sense that we should only add it as temporary if a copy was done and the pointer changed.

[zcbenz]

Yeah I got the cause wrong, it turns out to be the implicit copy in data()..

[angeloskath]

Nice! That makes a lot of sense and it is a sneaky one hidden in plain sight.