[Dy2static] Auto Remove Step Scope while GradRunProgramNode GCed.

[2742195759]

PR types

Performance optimization

PR changes

Others

Describe

PCard-66972
用户可以支持如下的操作，而不用显示的切换到eval模式下了：

import paddle
import time
def func(x):
    return  x * x

x = paddle.rand((1, 1000, 1000, 1000))
x.stop_gradient = True
func = paddle.jit.to_static(func)
for i in range(20):
    time.sleep(3)
    print("Step ", i)
    loss = func(x) # <--  mult-forward, but don't call backward.
    del loss

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[Aurelius84]

LGTM

[Aurelius84]

LGTM