Check variables were defined during block parsing

[wangkuiyi]

This discussion is related to #3708.

Think of the running process of the following example derived from the design of IfOp, where the global block has two sub-blocks, one as the left branch of an IfElseOp, the other as the right branch.

import paddle as pd

x1 = var()
x2 = var()

c = pd.create_ifelseop_builder(inputs=[x1, x2], output_num=2)
with c.true_block() as b:
    x1, x2 = b.inputs()
    z = mul(x1, x2))
    b.set_output(0, z)
    b.set_output(1, add(x1, x2))

with c.false_block() as b:
    x1, x2 = c1.inputs()
    b.set_output(0, add(x1, x2))
    b.set_output(1, mul(x1, x2))

cond = var()
out1, out2 = c(cond)

During the execution of above program, we want error as long as a line just executed refers to an undefined variable. For example, if we remove z = mul(x1, x2)) from above example, the execution of b.set_output(0, z) must report an error about "undefined variable z".

[wangkuiyi]

OK. Python would report such kind of errors. We don't need to do anything.

In particular, above example doesn't require that a block must record its parent block.