[MRG] Support RandomState instances in NumpyBackend.seed

[CunjieLee]

Types of changes

• Bug fix
• Tests
• Code cleanup
• New feature
• Documentation update

Motivation and context / Related issue

Closes #848.

NumpyBackend.seed() currently forwards every non-None value to
RandomState.seed(). Passing an existing np.random.RandomState
instance therefore raises:

TypeError: Cannot cast scalar from dtype('O') to dtype('int64')
according to the rule 'safe'

This forces callers that support both integer seeds and existing
RandomState instances to special-case the NumPy backend.

This change makes NumpyBackend adopt an externally supplied
RandomState, matching the generator-handling behavior of the other
backends. The NumPy-specific RandomState branches in the sliced
sampling utilities are removed accordingly.

The fourth occurrence mentioned in #848, in get_projections_spiral,
belongs to the still-open PR #838 and is not present on the current
master branch. That occurrence can use the unified backend path once
this fix is incorporated into #838.

How has this been tested (if it applies)

The issue was reproduced on the unmodified master branch using:

import numpy as np
from ot.backend import NumpyBackend

nx = NumpyBackend()
rng = np.random.RandomState(42)
nx.seed(rng)

The original code raises the reported TypeError.

The same code succeeds after the fix. Additional verification confirms
that:

• NumpyBackend.rng_ is the exact supplied RandomState instance;
• subsequent nx.randn(...) calls use that generator's sequence;
• integer seeding retains its previous deterministic behavior.

A regression test was added in test/test_backend.py.

The following checks pass:

pre-commit run --all-files
pytest --durations=20 -v test/ --doctest-modules

The full test run completed with 1456 passed, 96 skipped, and 4 xfailed.
The skipped tests require optional dependencies that are not installed in
the local test environment.

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (no public API documentation changes are required).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.