haiku: fully support JAX typed PRNG keys

For more details, see jax-ml/jax#17297. Previously, we had imagined a world where the jax_enable_custom_prng flag globally determined the presence of typed keys. This proved untenable for a number of reasons. Going forward, old-style and new-style keys are expected to exist side-by-side regardless of the value of `jax_enable_custom_prng`, which will soon be deprecated. Eventually old-style keys will also be deprecated and removed.

PiperOrigin-RevId: 565694585

haiku/_src/base.py

@@ -972,17 +972,24 @@ def assert_is_prng_key(key: PRNGKey):
   # device-to-host copy.
   make_error = lambda: ValueError(  # pylint: disable=g-long-lambda
       f"The provided key is not a JAX PRNGKey but a {type(key)}:\n{key}")
-  if jax.config.jax_enable_custom_prng:
-    if not isinstance(key, jax.random.KeyArray):
-      raise make_error()
+
+  if not hasattr(key, "shape") or not hasattr(key, "dtype"):
+    raise make_error()
+
+  if hasattr(jax.dtypes, "prng_key"):  # JAX 0.4.14 or newer
+    is_typed_prng = jax.dtypes.issubdtype(key.dtype, jax.dtypes.prng_key)
+  elif hasattr(jax.random, "PRNGKeyArray"):  # Older JAX versions
+    is_typed_prng = isinstance(key, jax.random.PRNGKeyArray)
+  else:  # Shouldn't get here, but just in case...
+    is_typed_prng = False
+
+  if is_typed_prng:
     if key.shape:
       raise ValueError(
           "Provided key did not have expected shape and/or dtype: "
           f"expected=(shape=(), dtype={key.dtype}), "
           f"actual=(shape={key.shape}, dtype={key.dtype})")
   else:
-    if not hasattr(key, "shape") or not hasattr(key, "dtype"):
-      raise make_error()
     config_hint = ""
     default_impl = jax.random.default_prng_impl()
     expected_shape = default_impl.key_shape

haiku/_src/random_test.py

@@ -75,14 +75,6 @@ def test_rbg_default_impl_invalid_key_shape(self):
 
 class CustomRNGTest(parameterized.TestCase):
 
-  def setUp(self):
-    super().setUp()
-    jax.config.update("jax_enable_custom_prng", True)
-
-  def tearDown(self):
-    super().tearDown()
-    jax.config.update("jax_enable_custom_prng", False)
-
   def test_non_custom_key(self):
     init, _ = transform.transform(base.next_rng_key)
     init(jax.random.PRNGKey(42))  # does not crash
@@ -116,11 +108,6 @@ def count_splits(_, num):
     init(key)
     self.assertEqual(count, 1)
 
-  def test_invalid_custom_key(self):
-    init, _ = transform.transform(base.next_rng_key)
-    with self.assertRaisesRegex(ValueError, "Init must be called with an RNG"):
-      init(jnp.ones((2,), dtype=jnp.uint32))
-
 
 def split_for_n(key, n):
   for _ in range(n):