shard_map: add API docs

docs/jax.experimental.rst

@@ -26,6 +26,7 @@ Experimental Modules
     jax.experimental.compilation_cache
     jax.experimental.key_reuse
     jax.experimental.mesh_utils
+    jax.experimental.shard_map
 
 Experimental APIs
 -----------------

docs/jax.experimental.shard_map.rst

@@ -0,0 +1,12 @@
+``jax.experimental.shard_map`` module
+=====================================
+
+.. automodule:: jax.experimental.shard_map
+
+API
+---
+
+.. autosummary::
+  :toctree: _autosummary
+
+  shard_map

jax/experimental/shard_map.py

@@ -20,7 +20,7 @@ import inspect
 import itertools as it
 from math import prod
 import operator as op
-from typing import Any, Callable, Optional, TypeVar, Union
+from typing import Any, Callable, TypeVar, Union
 
 import numpy as np
 
@@ -29,7 +29,6 @@ import jax.numpy as jnp
 from jax.sharding import NamedSharding, PartitionSpec, Mesh
 from jax._src import ad_checkpoint
 from jax._src import ad_util
-from jax._src import array
 from jax._src import callback
 from jax._src import core
 from jax._src import custom_derivatives
@@ -52,8 +51,7 @@ from jax._src.lax import (lax, parallel as lax_parallel, slicing,
                           special, control_flow, ann)
 from jax._src.util import (HashableFunction, HashablePartial, unzip2, unzip3,
                            as_hashable_function, memoize, partition_list,
-                           merge_lists, split_list, subs_list2,
-                           weakref_lru_cache)
+                           merge_lists, split_list, subs_list2)
 from jax.api_util import flatten_fun_nokwargs, shaped_abstractify
 from jax._src.interpreters import batching
 from jax._src.interpreters import mlir
@@ -82,6 +80,56 @@ AxisName = Hashable
 @traceback_util.api_boundary
 def shard_map(f: Callable, mesh: Mesh, in_specs: Specs, out_specs: Specs,
               check_rep: bool = True, auto: frozenset[AxisName] = frozenset()):
+  """Map a function over shards of data.
+
+  Note:
+    ``shard_map`` is an experimental API, and still subject to change. For an
+    introduction to sharded data, refer to :ref:`sharded-computation`. For a more
+    in-depth look at using ``shard_map``, refer to `SPMD multi-device parallelism with shard_map`_.
+
+  Args:
+    f: callable to be mapped. Each application of ``f``, or "instance" of ``f``,
+      takes as input a shard of the mapped-over arguments and produces a shard
+      of the output.
+    mesh: a ``jax.sharding.Mesh`` representing the array of devices over which
+      to shard the data and on which to execute instances of ``f``. The names of
+      the ``Mesh`` can be used in collective communication operations in ``f``.
+      This is typically created by a utility function like
+      :func:`jax.experimental.mesh_utils.create_device_mesh`.
+    in_specs: a pytree with :class:`~jax.sharding.PartitionSpec` instances as leaves,
+      with a tree structure that is a tree prefix of the args tuple to be mapped
+      over. Similar to :class:`~jax.sharding.NamedSharding`, each ``PartitionSpec``
+      represents how the corresponding argument (or subtree of arguments) should
+      be sharded along the named axes of ``mesh``. In each ``PartitionSpec``,
+      mentioning a ``mesh`` axis name at a position expresses sharding the
+      corresponding argument array axis along that positional axis; not
+      mentioning an axis name expresses replication.
+    out_specs: a pytree with :class:`~jax.sharding.PartitionSpec` instances as leaves,
+      with a tree structure that is a tree prefix of the output of ``f``. Each
+      ``PartitionSpec`` represents how the corresponding output shards should be
+      concatenated. In each ``PartitionSpec``, metioning a ``mesh`` axis name at
+      a position expresses concatenation of that mesh axis's shards along the
+      corresponding positional axis. Not mentioning a ``mesh`` axis name
+      expresses a promise that the output values are equal along that mesh axis,
+      and that rather than concatenating only a single value should be produced.
+    check_rep: If True (default) enable additional validity checks and automatic
+      differentiation optimizations. The validity checks concern whether any mesh
+      axis names not mentioned in ``out_specs`` are consistent with how the outputs
+      of ``f`` are replicated. Must be set False if using a Pallas kernel in ``f``.
+    auto: (experimental) an optional set of axis names from ``mesh`` over which we
+      do not shard the data or map the function, but rather we allow the
+      compiler to control sharding. These names cannot be used in ``in_specs``,
+      ``out_specs``, or in communication collectives in ``f``.
+
+  Returns:
+    A callable that applies the input function ``f`` across data sharded according to
+    the ``mesh`` and ``in_specs``.
+
+  Examples:
+    For examples, refer to :ref:`sharded-computation` or `SPMD multi-device parallelism with shard_map`_.
+
+  .. _SPMD multi-device parallelism with shard_map: https://jax.readthedocs.io/en/latest/notebooks/shard_map.html
+  """
   return _shard_map(f, mesh, in_specs, out_specs, check_rep, auto)
 
 def _shard_map(f: Callable, mesh: Mesh, in_specs: Specs,