Implement dpnp.ndarray.__iter__ method (#2206)

The PR proposes to add implementation of `dpnp.ndarray.__iter__` method.
The new code is assumed to be covered by third party tests.

- [x] Have you provided a meaningful PR description?
- [ ] Have you added a test, reproducer or referred to issue with a
reproducer?
- [x] Have you tested your changes locally for CPU and GPU devices?
- [x] Have you made sure that new changes do not introduce compiler
warnings?
- [ ] Have you checked performance impact of proposed changes?
- [ ] If this PR is a work in progress, are you filing the PR as a
draft?

dpnp/dpnp_array.py

@@ -434,7 +434,11 @@ def __isub__(self, other):
         dpnp.subtract(self, other, out=self)
         return self
 
-    # '__iter__',
+    def __iter__(self):
+        """Return ``iter(self)``."""
+        if self.ndim == 0:
+            raise TypeError("iteration over a 0-d array")
+        return (self[i] for i in range(self.shape[0]))
 
     def __itruediv__(self, other):
         """Return ``self/=value``."""

dpnp/tests/third_party/cupy/core_tests/test_iter.py

@@ -0,0 +1,44 @@
+import unittest
+
+import numpy
+import pytest
+
+import dpnp as cupy
+from dpnp.tests.third_party.cupy import testing
+
+
+@testing.parameterize(
+    *testing.product(
+        {"shape": [(3,), (2, 3, 4), (0,), (0, 2), (3, 0)]},
+    )
+)
+class TestIter(unittest.TestCase):
+
+    @testing.for_all_dtypes()
+    @testing.numpy_cupy_array_equal()
+    def test_list(self, xp, dtype):
+        x = testing.shaped_arange(self.shape, xp, dtype)
+        return list(x)
+
+    @testing.for_all_dtypes()
+    @testing.numpy_cupy_equal()
+    def test_len(self, xp, dtype):
+        x = testing.shaped_arange(self.shape, xp, dtype)
+        return len(x)
+
+
+class TestIterInvalid(unittest.TestCase):
+
+    @testing.for_all_dtypes()
+    def test_iter(self, dtype):
+        for xp in (numpy, cupy):
+            x = testing.shaped_arange((), xp, dtype)
+            with pytest.raises(TypeError):
+                iter(x)
+
+    @testing.for_all_dtypes()
+    def test_len(self, dtype):
+        for xp in (numpy, cupy):
+            x = testing.shaped_arange((), xp, dtype)
+            with pytest.raises(TypeError):
+                len(x)