rust: interpret summaries with no value as null tensors (#4735)
rust: interpret summaries with no value as null tensors (#4735)

* rust: add SummaryWriterExt write_tensor() method

* rust: implement data_compat.rs basic tensor support

* rust: make RunLoader enrich and commit tensors

* rust: interpret summaries with no value as null tensors

* cr: use stated dtype for null_tensor_proto()
tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard