Created by: typhoonzero

Inspired by https://github.com/PaddlePaddle/Paddle/issues/6517#issuecomment-351935737

Should we have blocks inside operators? For example, in programming languages, some statements controls, and evaluates several blocks:

if (condition statement) {
   true condition block
} else {
  false condition block
}

Or

while (condition statement) {
  loop block
}

For now, we implement this by putting a BlockDescBind inside the operator's Attr and use Executor to run the block inside operator. The Executor is created for multiple times and runs unpredictable inside operators.

In that case, we can make this while_op and if_else_op bind with Executor to run:

var1 = layers.var()
var2 = layers.var()
with if_statement(math.ops.greater(var1, var2)) as ifop:
    # define subblock layers

op:
  name: if_else
  blocks:
    block 0: condition
    block 1: true block
    block 2: else block

Then the Executor will do differently control statements if the operator type is while or if.