docs(mge/distributed): add distributed.helper docs

GitOrigin-RevId: 37c14aa11f7514d6c6c7d2051d9f11c6b261a6fa

imperative/python/megengine/distributed/helper.py

@@ -21,7 +21,7 @@ from ..functional.param_pack import get_offsets, pack_allreduce_split
 from ..functional.utils import copy
 from ..utils.future import Future
 from .functional import all_reduce_sum, broadcast
-from .group import WORLD, group_barrier, is_distributed
+from .group import WORLD, Group, group_barrier, is_distributed
 
 
 class TensorFuture(Future):
@@ -54,28 +54,43 @@ def synchronized(func: Callable):
     return wrapper
 
 
-def worker(queue, device_type):
+def _get_device_count_worker(queue, device_type):
     num = get_device_count(device_type)
     queue.put(num)
 
 
 def get_device_count_by_fork(device_type: str):
+    """Get device count in fork thread.
+    See https://stackoverflow.com/questions/22950047/cuda-initialization-error-after-fork
+    for more information.
+    """
     q = mp.Queue()
-    p = mp.Process(target=worker, args=(q, device_type))
+    p = mp.Process(target=_get_device_count_worker, args=(q, device_type))
     p.start()
     p.join()
     return q.get()
 
 
-def bcast_list_(params, group):
-    for p in params:
-        p._reset(broadcast(p, group))
+def bcast_list_(inps: list, group: Group = WORLD):
+    """Broadcast tensors between given group.
+
+    :param inps: input tensors.
+    :param group: communication group.
+    """
+    for inp in inps:
+        inp._reset(broadcast(inp, group))
 
 
 class AllreduceCallback:
-    def __init__(self, reduce_method, group=WORLD):
+    """Allreduce Callback with tensor fusion optimization.
+
+    :param reduce_method: the method to reduce gradiants.
+    :param group: communication group.
+    """
+
+    def __init__(self, reduce_method: str, group: Group = WORLD):
         reduce_method = reduce_method.lower()
-        assert reduce_method in ["sum", "mean"]
+        assert reduce_method in ["sum", "mean"], "reduce_method should be sum or mean"
         self._reduce_method = reduce_method
         self._group = group
         self._marked_gm = WeakSet()