* Add support for None for fluid.data (#20228)

* Refine Error Message for New Data API (#20204)

* Modify English doc of fluid.data (#20248)

* Add lod_level parameter to paddle.fluid.data

* Add "optional" in API doc for optional parameters

Set output type LoDTensor only

After code experiment, I found data doens't support other type

The new "fluid.data" changes old "fluid.layers.data":

1. Add shape and dtype check.
2. Remove "append_batch_size" parameter. We won't offer this in the new data layer because other deep learning platforms don't have this kind of data layer pre-processing. It may confuse users.
3. Remove "stop gradient" parameter because the data layer doesn't do back-propagation

TODO：
Now data layer feeded by executor is checked, will we want to check the feed data of readers in the future?