Keras-like API Some layers and LeNet example (#2214)
Keras-like API Some layers and LeNet example (#2214)

* update dense

* style

* doc for conv2d

* add error msg

* add ser test

* fix softmax

* readme for lenet

* remove

* fix style

* softmax 2d ut

* add ser test for softmax

* fix grammar

* review and update

* scala doc

* fix

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