Convert IFRT's kString dtype to NumPy's variable length string dtype.

Previously, `xla::ifrt::DType::kString` would be translated to
`NPY_OBJECT`. NumPy 2.0 has introduced a variable length string type,
`NPY_VSTRING`, so convert to that when available. This is consistent
with `xla::DtypeToIfRtDType`, which performs the opposite translation.

PiperOrigin-RevId: 813472617

Author: Google-ML-Automation

tests/string_array_test.py

@@ -62,6 +62,7 @@ def test_single_device_array(self, asarray):
       jax_string_array = jax.device_put(
           numpy_string_array, device=cpu_devices[0]
       )
+    self.assertEqual(jax_string_array.dtype, np.dtypes.StringDType())  # type: ignore
     jax_string_array.block_until_ready()
 
     array_read_back = jax.device_get(jax_string_array)
@@ -93,8 +94,13 @@ def test_multi_device_array(self, asarray):
       jax_string_array = jnp.asarray(numpy_string_array, device=sharding)
     else:
       jax_string_array = jax.device_put(numpy_string_array, device=sharding)
+    self.assertEqual(jax_string_array.dtype, np.dtypes.StringDType())  # type: ignore
     jax_string_array.block_until_ready()
 
+    self.assertLen(jax_string_array._arrays, 2)
+    for a in jax_string_array._arrays:
+      self.assertEqual(a.dtype, np.dtypes.StringDType())  # type: ignore
+
     array_read_back = jax.device_get(jax_string_array)
     self.assertEqual(array_read_back.dtype, np.dtypes.StringDType())  # type: ignore
     np.testing.assert_array_equal(array_read_back, numpy_string_array)