InputSpec = function InputSpec(args) {
	  this.dtype = args.dtype;
	  this.shape = args.shape;
	  /*
	    TODO(michaelterry): Could throw error if ndim and shape are both defined
	      (then backport).
	  */

	  if (args.shape != null) {
	    this.ndim = args.shape.length;
	  } else {
	    this.ndim = args.ndim;
	  }

	  this.maxNDim = args.maxNDim;
	  this.minNDim = args.minNDim;
	  this.axes = args.axes || {};
	}
Specifies the ndim, dtype and shape of every input to a layer.

Every layer should expose (if appropriate) an `inputSpec` attribute:
a list of instances of InputSpec (one per input tensor).

A null entry in a shape is compatible with any dimension,
a null shape is compatible with any shape.