add metric for mnist.

Add Layer.__init__ in Input.__init__

Refine Loss in Model

To make save/load compatible between dygraph and static-graph.

2. prune unraleted ops in test program

* 1. remove shape_hints and add Input
* 2. remove infer_shape and infer_dtype in Loss
* 3. update mnist example, see it for usage

Add model API and demo