docs(mge/param_pack): add description of param pack and add param pack in example run_dist

GitOrigin-RevId: 1c900e159d043371a61479cb414b14bf31f48943

python_module/megengine/module/parampack.py

@@ -16,6 +16,14 @@ from .._internal.opr import param_pack_split
 
 
 class ParamPack(Module):
+    r"""Pack module's parameters
+
+    :param model: the module you want to pack parameters.
+    :param nr_ignore_first: how many parameters will be unpacked at first.
+    :param max_size_per_group: upper bound of packed parameters' size in MB.
+    :param max_nr_params_per_group: upper bound of the number of parameters of each group.
+
+    """
     def __init__(self,
                  model: Module,
                  nr_ignore_first:int = 8,