Implement Select op.

[kavyasrinet]

Implementation of Select operation in Paddle, similar to how it is implemented in GoLang.
A description of the functionality is here : #7620 and the design of the Python API has been proposed here: https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/csp.md

Please feel free to add more relevant resources as you see fit.

[kavyasrinet]

As of now this resource suggests one way to think about the implementation: https://github.com/tylertreat/chan/blob/b4120abe5d642284182f04965e793cc918daf711/src/chan.c

[abhinavarora]

The Select will be implemented in a fashion that is similar to Go. Go's implementation of Select is described in http://www.tapirgames.com/blog/golang-concurrent-select-implementation.

[shanyi15]

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