Add math.flatten (#229)
Add math.flatten (#229)

* added math flatten

* moved flatten op to ndarray

tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs-core,tensorflow/tfjs,tensorflow/tfjs-core,tensorflow/tfjs-core