classify api of mindspore.train by submodule

Signed-off-by: NTing Wang <kathy.wangting@huawei.com>

api/source_en/api/python/mindspore/mindspore.train.rst

 mindspore.train
 ===============
 
+mindspore.train.summary
+-----------------------
+
 .. automodule:: mindspore.train.summary
     :members:
 
+mindspore.train.callback
+------------------------
+
 .. automodule:: mindspore.train.callback
     :members:
 
+mindspore.train.serialization
+-----------------------------
+
 .. automodule:: mindspore.train.serialization
     :members:
 
+mindspore.train.amp
+-------------------
+
 .. automodule:: mindspore.train.amp
     :members:
 
+mindspore.train.loss_scale_manager
+----------------------------------
+
 .. automodule:: mindspore.train.loss_scale_manager
     :members:
\ No newline at end of file

api/source_zh_cn/api/python/mindspore/mindspore.train.rst

 mindspore.train
 ===============
 
+mindspore.train.summary
+-----------------------
+
 .. automodule:: mindspore.train.summary
     :members:
 
+mindspore.train.callback
+------------------------
+
 .. automodule:: mindspore.train.callback
     :members:
 
+mindspore.train.serialization
+-----------------------------
+
 .. automodule:: mindspore.train.serialization
     :members:
 
+mindspore.train.amp
+-------------------
+
 .. automodule:: mindspore.train.amp
     :members:
 
+mindspore.train.loss_scale_manager
+----------------------------------
+
 .. automodule:: mindspore.train.loss_scale_manager
     :members:
\ No newline at end of file