Add async update design doc

doc/fluid/design/dist_train/async_update.md

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+# Design Doc: Asynchronous Update With Distributed Training
+
+## Background
+
+For the typical synchronous distributed training, some significant steps are as follows:
+
+1. A Trainer will compute the gradients and SEND them to the Parameter Server(PServer) nodes.
+1. After the PServer node received gradients came from all the Trainers, It will aggregate the
+gradient variables for the same parameter into one gradient variable and then apply the aggregated
+gradient to the respective parameter, finally using an optimize algorithms(SGD, Monument...)
+to update the parameters.
+1. The Trainer would wait for the PServers finished the optimize stage, and GET the parameters from PServer,
+so all the Trainers would get the same parameters.
+
+In the synchronously distributed training, there should be a `Barrier` to synchronise the
+parameters after the optimizing stage. The performance of a distributed training job would
+depend on the slowest node if there were hundreds or thousands of training nodes in a
+Job, the performance of synchronously distributed training might be very poor because of
+the slow node. So this design doc would introduce an approach to implement
+*asynchronously* distributed training in PaddlePaddle Fluid.
+
+## Design
+
+<img src="./src/async_update.png" width="600"/>
+
+As the figure above, we describe a global view of asynchronously update process and use
+the parameter `w1` as an example to introduce the steps:
+1. For each gradient variables, they may distribute on different GPU card and aggregate
+them while they are all calculated.
+1. Split the gradient variable into multiple blocks according to the number of PServer
+instances and then send them.
+1. PServer would run an `Optimize Block` using a specified optimize algorithm to update
+the specified parameter.
+1. The trainer will fetch latest parameter from PServer before running forward Op which depends
+on the specified parameter.
+1. Broadcast the received variable into multiple GPU cards and continue to run the next
+mini-batch.
+
+### Trainer
+
+- For the multiple devices distributed training, we need to aggregate the gradient
+variables which placed on different devices firstly and then schedule a `SendVars` Operator to
+send the gradient variables to the multiple PServer instances.
+- Schedule `FetchVars` operator to fetch the latest parameter from PServer before running
+the forward ops.
+- There could be a large number of gradient variables to be sent, so we need to use another
+thread pool(IO Threadpool) whose a number of the schedulable threads is larger than the
+computing thread pool to avoid competitive the thread resources with computing.
+
+### Parameter Server
+
+<img src="./src/async_pserver.png" width="750"/>
+
+- There should be multiple trainer instances want to optimize the same parameter at
+the same time, to avoid the racing, we need one `BlockingQueue` for each gradient
+variable to process them one by one.
+- We need a `Map` structure to map a gradient variable name to the `OptimizeBlock` which
+can optimize the respective parameter.