[JAX] Update users of jax.tree.map() to be more careful about how the…

…y handle Nones.

Due to a bug in JAX, JAX previously permitted `jax.tree.map(f, None, x)` where `x` is not `None`, effectively treating `None` as if it were pytree-prefix of any value. But `None` is a pytree container, and it is only a prefix of `None` itself.

Fix user code that was relying on this bug. Most commonly, the fix is to write
`jax.tree.map(lambda a, b: (None if a is None else f(a, b)), x, y, is_leaf=lambda t: t is None)`.

PiperOrigin-RevId: 641687779

clrs/_src/baselines.py

@@ -766,7 +766,9 @@ def _keep_in_algo(k, v):
     masked_grads = grads
   else:
     masked_grads = {k: _keep_in_algo(k, v) for k, v in grads.items()}
-  flat_grads, treedef = jax.tree_util.tree_flatten(masked_grads)
+  flat_grads, treedef = jax.tree_util.tree_flatten(
+      masked_grads, is_leaf=lambda x: x is None
+  )
   flat_opt_state = jax.tree_util.tree_map(
       lambda _, x: x  # pylint:disable=g-long-lambda
       if isinstance(x, (np.ndarray, jax.Array))