Implement flatten one level with keys in C++ and use it for the prefix/equality error printing.

With this, we should be able to safely delete the python with-path registry after a new jaxlib release.

Also changed all `std::string_view` to `absl::string_view` per requirements of TF repository.

PiperOrigin-RevId: 705669465

jax/_src/tree_util.py

@@ -25,6 +25,7 @@ from typing import Any, NamedTuple, TypeVar, overload
 
 from jax._src import traceback_util
 from jax._src.lib import pytree
+from jax._src.lib import xla_extension_version
 from jax._src.util import safe_zip, set_module
 from jax._src.util import unzip2
 
@@ -607,6 +608,18 @@ def flatten_one_level(tree: Any) -> tuple[Iterable[Any], Hashable]:
     return out
 
 
+# flatten_one_level_with_keys is not exported.
+def flatten_one_level_with_keys(
+    tree: Any,
+) -> tuple[Iterable[KeyLeafPair], Hashable]:
+  """Flatten the given pytree node by one level, with keys."""
+  out = default_registry.flatten_one_level_with_keys(tree)
+  if out is None:
+    raise ValueError(f"can't tree-flatten type: {type(tree)}")
+  else:
+    return out
+
+
 # prefix_errors is not exported
 def prefix_errors(prefix_tree: Any, full_tree: Any,
                   is_leaf: Callable[[Any], bool] | None = None,
@@ -728,7 +741,7 @@ def keystr(keys: KeyPath):
   return ''.join(map(str, keys))
 
 
-# TODO(ivyzheng): remove this after _child_keys() also moved to C++.
+# TODO(ivyzheng): remove this after another jaxlib release.
 class _RegistryWithKeypathsEntry(NamedTuple):
   flatten_with_keys: Callable[..., Any]
   unflatten_func: Callable[..., Any]
@@ -1146,6 +1159,8 @@ def tree_map_with_path(f: Callable[..., Any],
 
 def _child_keys(pytree: Any) -> KeyPath:
   assert not treedef_is_strict_leaf(tree_structure(pytree))
+  if xla_extension_version >= 301:
+    return tuple(k for k, _ in flatten_one_level_with_keys(pytree)[0])
   handler = _registry_with_keypaths.get(type(pytree))
   if handler:
     return tuple(k for k, _ in handler.flatten_with_keys(pytree)[0])