remove vars when remove ops

[luotao1]

when removing an op, the vars of this op should be removed at the same time:

• output vars of this op: remove all.
• input vars of this op: remove the input vars which are not used by other ops.

add a simple unit test to check RemoveOp method.

[typhoonzero]

Is there a case that the output of the removed op is used by other op?

[luotao1]

If the output of removed op is the input of other ops, then other ops should adjust their inputs. If the output of removed op is the output of other ops, although this case has not been seen now, but these output vars should be remained, how do you think about it ?

[qingqing01]

Is there a case that the output of the removed op is used by other op?

I think this case is in existence. Maybe two ways to remove var:

1. remove vars specified by name ?
2. go through ProgramDesc to check whether some unused vars need to be removed ?

[typhoonzero]

If we want to make this a general function, it's quite complex, see #9080 and https://en.wikipedia.org/wiki/Static_single_assignment_form. We must build a SSA graph before head so that two ops have same outputs ( write to var ) won't affect each other.

For the simple implementation, I think just check if the output is used by other ops and do not remove them should work.

@qingqing01

remove vars specified by name ?

For generate inference programdesc, if remove_op can also remove unused vars, it's simpler to transpile a program for inference, I think.

[luotao1]

I agree with the simple implementation for generating inference program-desc at first.
And I adjust the remove rule of variables: for either input or output variable, if it is also an input or output variable of other ops, we should remain it.

[typhoonzero]

LGTM++