Return tuple instead of list from atleast_1d/2d/3d and ogrid

[antonwolfy]

NumPy returns a tuple of arrays (not a list) from several functions that yield multiple arrays.
This PR aligns dpnp with that behavior.

What is done

• dpnp.atleast_1d, dpnp.atleast_2d, dpnp.atleast_3d and dpnp.ogrid now return a tuple of arrays instead of a list. Single-input
  atleast_*d and single-slice ogrid still return a bare array, and mgrid still returns a single stacked array (matching NumPy).
• Added/extended tests to assert the return container type matches NumPy. While here, added the same parity checks for other tuple-returning functions that lacked them (nonzero, where, ix_, tril_indices, triu_indices, unravel_index, unstack).

The split family (array_split, split, hsplit, vsplit, dsplit) and histogramdd still return lists, matching current NumPy.

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[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2965/index.html

[coveralls]

coverage: 78.069% (+0.003%) from 78.066% — return-tuple-of-array-acc-to-numpy-2.0 into master

[github-actions]

Array API standard conformance tests for dpnp=0.21.0dev1=py313h509198e_28 ran successfully.
Passed: 1355
Failed: 5
Skipped: 16

[ndgrigorian]

LGTM