Create go_op design doc (#9389)

* Create go_op design doc

doc/fluid/design/concurrent/go_op.md

+# go_op Design
+
+## Introduction
+
+The **go_op** allows user's of PaddlePaddle to run program blocks on a detached
+thread.  It works in conjuction with CSP operators (channel_send, 
+channel_receive, channel_open, channel_close, and select) to allow users to
+concurrently process data and communicate easily between different threads.
+
+## How to use it
+
+```
+channel = fluid.make_channel(dtype=core.VarDesc.VarType.LOD_TENSOR)
+
+with fluid.Go():
+    # Send a tensor of value 99 to "channel" on a detached thread
+    tensor = fill_constant(shape=[1], dtype='int', value=99)
+    tensor.stop_gradient = True
+    fluid.channel_send(channel, tensor)
+    
+# Receive sent tensor from "channel" on the main thread
+result = fill_constant(shape=[1], dtype='int', value=-1)    
+fluid.channel_recv(ch, result)  
+```
+
+The go operator can be accessed by using the fluid.Go() control flow.  This
+will create a new sub block, where the user can add additional operators
+to be ran on the thread.
+
+**Note:** Since back propegation is currently not support in the go_op, users
+should ensure that operators in the go block does not require gradient 
+calculations.
+
+## How it Works
+
+Similar to other control blocks, go_op will create a sub block and add it
+as a child to the current block.  Operators and variables defined in this
+block will be added to the go sub_block.
+
+In addition, the go operator will create a new child scope whose parent is
+the global scope.  Please refer to [block captures](#block-captures) for more
+information.
+
+When Paddle executor runs go_op, go_op will take the sub_block and pass it to
+the executor.run method (along with a newly created local scope) on a detached
+thread.
+
+An example of the generated program description is shown below.  Take note of
+the **go_op** in particular.  It is added as an operator in the current 
+block (in this example, block0).  The **go_op** contains a `sub_block`
+attribute, which points to the id of the block that will be executed in a 
+detached thread.
+
+```
+blocks {
+  idx: 0
+  parent_idx: -1
+  vars {
+    name: "return_value"
+    type {
+      type: LOD_TENSOR
+      lod_tensor {
+        tensor {
+          data_type: INT64
+        }
+      }
+    }
+  }
+  vars {
+    name: "status_recv"
+    type {
+      type: LOD_TENSOR
+      lod_tensor {
+        tensor {
+          data_type: BOOL
+        }
+      }
+    }
+  }
+  ...
+  ops {
+    outputs {
+      parameter: "Out"
+      arguments: "channel"
+    }
+    type: "channel_create"
+    attrs {
+      name: "data_type"
+      type: INT
+      i: 7
+    }
+    attrs {
+      name: "capacity"
+      type: INT
+      i: 0
+    }
+  }
+  ops {
+    inputs {
+      parameter: "X"
+      arguments: "channel"
+    }
+    type: "go"
+    attrs {
+      name: "sub_block"
+      type: BLOCK
+      block_idx: 1
+    }
+  }
+  ops {
+    inputs {
+      parameter: "Channel"
+      arguments: "channel"
+    }
+    outputs {
+      parameter: "Out"
+      arguments: "return_value"
+    }
+    outputs {
+      parameter: "Status"
+      arguments: "status_recv"
+    }
+    type: "channel_recv"
+  }
+  ...
+}
+
+blocks {
+  idx: 1
+  parent_idx: 0
+  vars {
+    name: "status"
+    type {
+      type: LOD_TENSOR
+      lod_tensor {
+        tensor {
+          data_type: BOOL
+        }
+      }
+    }
+  }
+  ...
+  
+  ops {
+    outputs {
+      parameter: "Out"
+      arguments: "fill_constant_1.tmp_0"
+    }
+    type: "fill_constant"
+    attrs {
+      name: "force_cpu"
+      type: BOOLEAN
+      b: false
+    }
+    attrs {
+      name: "value"
+      type: FLOAT
+      f: 99.0
+    }
+    attrs {
+      name: "shape"
+      type: INTS
+      ints: 1
+    }
+    attrs {
+      name: "dtype"
+      type: INT
+      i: 3
+    }
+  }
+  ops {
+    inputs {
+      parameter: "Channel"
+      arguments: "channel"
+    }
+    inputs {
+      parameter: "X"
+      arguments: "fill_constant_1.tmp_0"
+    }
+    outputs {
+      parameter: "Status"
+      arguments: "status"
+    }
+    type: "channel_send"
+    attrs {
+      name: "copy"
+      type: BOOLEAN
+      b: false
+    }
+  }
+```
+
+## Current Limitations
+
+#### <a name="block-captures"></a>Scopes and block captures:
+
+Paddle utilizes [scopes](./../concepts/scope.md) to store variables used in a
+block.  When a block is executed, a new local scope is created from the parent
+scope (ie: scope derived from the parent block) and associated with the new 
+child block.  After the block finishes executing, then the local scope and
+all associated variables in the scope is deleted.
+
+This works well in a single threaded scenario, however with introduction of
+go_op, a child block may continue to execute even after the parent block has
+exited.  If the go_op tries to access variables located in the parent block's
+scope, it may receive a segmentation fault because the parent scope may have
+been deleted.
+
+We need to implement block closures in order to prevent access to parent
+scope variables from causing a segmentation fault.  As a temporary workaround,
+please ensure that all variables accessed in the go block is not destructed
+before it is being accessed.  Currently, the go_op will explicitly enforce 
+this requirement and raise an exception if a variable could not be found in 
+the scope.
+
+Please refer to [Closure issue](https://github.com/PaddlePaddle/Paddle/issues/8502)
+for more details.
+
+#### Green Threads
+
+Golang utilizes `green threads`, which is a mechnism for the runtime library to 
+manage multiple threads (instead of natively by the OS).  Green threads usually
+allows for faster thread creation and switching, as there is less overhead
+when spawning these threads.  For the first version of CSP, we only support
+OS threads.
+
+
+#### Backward Propegation:
+
+go_op currently does not support backwards propagation.  Please use go_op with
+non training operators.