Move transfer python bindings into jax.

PiperOrigin-RevId: 719082208

jax/BUILD

@@ -798,6 +798,15 @@ pytype_library(
     ],
 )
 
+pytype_library(
+    name = "experimental_transfer",
+    srcs = ["experimental/transfer.py"],
+    deps = [
+        ":jax",
+        "//jax/_src/lib",
+    ],
+)
+
 pytype_strict_library(
     name = "pickle_util",
     srcs = ["_src/pickle_util.py"],

jax/experimental/transfer.py

@@ -0,0 +1,74 @@
+# Copyright 2025 The JAX Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""`jax.experimental.transfer`: DCN cross slice transfer."""
+
+import jax
+from typing import Any, TYPE_CHECKING
+from jax._src.lib import xla_client as _xc
+from jax._src.lib import xla_extension_version
+from jax._src.util import use_cpp_class, use_cpp_method
+
+class TransferConnection:
+  """Represents a connection to exactly one peer."""
+
+  @use_cpp_method()
+  def _pull_flat(self, uuid, backend, xs_flat):
+    raise NotImplementedError()
+
+  def pull(self, uuid: int, xs: Any) -> Any:
+    """Fetches a pytree of arrays from a remote device.
+
+    Args:
+       uuid: identifier for the request
+       xs: A pytree of ShapeDtypeStruct.
+    Returns:
+       A pytree of arrays.
+    """
+    xs_flat, tree = jax.tree.flatten(xs)
+    if not xs_flat:
+      return xs
+    backend = next(iter(xs_flat[0].sharding.device_set)).client
+    return tree.unflatten(self._pull_flat(uuid, backend, xs_flat))
+
+
+if not TYPE_CHECKING and xla_extension_version >= 305:
+  TransferConnection = use_cpp_class(_xc._xla.TransferConnection)(TransferConnection)
+
+
+class TransferServer:
+
+  @use_cpp_method()
+  def address(self) -> str:
+    """Returns the address that this server can be connected to with."""
+    raise NotImplementedError()
+
+  @use_cpp_method()
+  def _await_pull_flat(self, uuid, args: list[jax.Array]):
+    raise NotImplementedError()
+
+  @use_cpp_method()
+  def connect(self, address: str) -> TransferConnection:
+    """Creates a connection to a remote server."""
+    raise NotImplementedError()
+
+  def await_pull(self, uuid: int, arrays: Any) -> Any:
+    """Schedules a pytree of arrays to be fetched by a remote device."""
+    self._await_pull_flat(uuid, jax.tree.flatten(arrays)[0])
+
+
+if not TYPE_CHECKING and xla_extension_version >= 305:
+  TransferServer = use_cpp_class(_xc._xla.TransferServer)(TransferServer)
+
+if xla_extension_version >= 305:
+  start_transfer_server = _xc._xla.start_transfer_server