Improves numerical precision (if applicable) using pairwise summation:
https://en.wikipedia.org/wiki/Pairwise_summation

Thanks to Rasmus Larsen for the succinct binary tree aggregation pseudocode.

Also adds AsString for Variant types: this emits the Variant as a string via
its DebugString().

PiperOrigin-RevId: 340246073
Change-Id: I009281f46cbea30d6e33ecf79a1723d62e96cc6d

new op_requires.h

PiperOrigin-RevId: 295819544
Change-Id: If9ac368d7bcc0eadca38a73aff2f86dd0220c87a

OpKernel{,Construction,Context} to decouple this kernel from TensorFlow's
implementations of these data structures.

PiperOrigin-RevId: 294258181
Change-Id: I0a4610857e2681cddb0b96b8e0632244633f6315

PiperOrigin-RevId: 209679086

Change: 123900938

Changes:

* error message that refers to removed `DefaultSession` method.
* -Wnull-conversion warnings
* the "_start_time" attr for recvs when the flag "--brain_enable_scheduling_for_recvs" is set.
* typo in tutorial data download progress message.
* a typo ("however their installing"=>"however installing").
* typo, rename "TensorFlow Mechanics" to "How To" to be consistent with the website.
* a typo ("subtact"=>"subtract").
* protobuf examples in comments in tensorflow::Example.proto.
* formula formatting in MNIST beginner tutorial
* negative fraction-of-queue-full stats
* protobuf inclusion path so that Android demo will build under Blaze.
* small typo (moderatly > moderately)
* Session.run() to check that tensor arguments come from the session's graph.
* another six import
* seq2seq typo in bazel command

Base CL: 108349164

error handling, updates to website.

Changes:
- Removes redundant reshape from image models by @mrry
- Default TensorBoard to localhost by @danmane
- Reformatting of tensorflow/core by @josh11b
- Make tutorials backwards compatible to 0.5.0 by @girving
- Improve print documentation (md files not updated).
- Add proper scrolling to sitemap by @martinwicke

Base CL: 107956254

TensorFlow is an open source software library for numerical computation
using data flow graphs.

Base CL: 107276108