add learning rate schedule to api docs

api/source_en/api/python/mindspore/mindspore.nn.learning_rate_schedule.rst

+mindspore.nn.learning_rate_schedule
+===================================
+
+.. automodule:: mindspore.nn.learning_rate_schedule
+   :members:
\ No newline at end of file

api/source_en/index.rst

@@ -17,6 +17,7 @@ MindSpore API
    api/python/mindspore/mindspore.context
    api/python/mindspore/mindspore.nn
    api/python/mindspore/mindspore.nn.dynamic_lr
+   api/python/mindspore/mindspore.nn.learning_rate_schedule
    api/python/mindspore/mindspore.ops
    api/python/mindspore/mindspore.ops.composite
    api/python/mindspore/mindspore.ops.operations

api/source_zh_cn/api/python/mindspore/mindspore.nn.learning_rate_schedule.rst

+mindspore.nn.learning_rate_schedule
+===================================
+
+.. automodule:: mindspore.nn.learning_rate_schedule
+   :members:

api/source_zh_cn/index.rst

@@ -17,6 +17,7 @@ MindSpore API
    api/python/mindspore/mindspore.context
    api/python/mindspore/mindspore.nn
    api/python/mindspore/mindspore.nn.dynamic_lr
+   api/python/mindspore/mindspore.nn.learning_rate_schedule
    api/python/mindspore/mindspore.ops
    api/python/mindspore/mindspore.ops.composite
    api/python/mindspore/mindspore.ops.operations