Add support for Tensors to the EventAccumulator.
Add support for Tensors to the EventAccumulator.

After this CL, it is possible to access TensorSummary data via the EventAccumulator and EventMultiplexer classes. This is a first step towards adding full TensorSummary support to TensorBoard.
Change: 148261489

ioeric/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,qiuminxu/tensorboard,agrubb/tensorboard,francoisluus/tensorboard-supervise,qiuminxu/tensorboard,agrubb/tensorboard,tensorflow/tensorboard,shakedel/tensorboard,shakedel/tensorboard,qiuminxu/tensorboard,qiuminxu/tensorboard,qiuminxu/tensorboard,ioeric/tensorboard,ioeric/tensorboard,ioeric/tensorboard,agrubb/tensorboard,tensorflow/tensorboard,shakedel/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,agrubb/tensorboard,shakedel/tensorboard,agrubb/tensorboard,francoisluus/tensorboard-supervise,francoisluus/tensorboard-supervise,francoisluus/tensorboard-supervise,ioeric/tensorboard,tensorflow/tensorboard,agrubb/tensorboard,qiuminxu/tensorboard,shakedel/tensorboard,francoisluus/tensorboard-supervise,francoisluus/tensorboard-supervise,shakedel/tensorboard,ioeric/tensorboard