fix bug when iterate in locals_var.items() in python3

[SunGaofeng]

for the codes in python/paddle/fluid/layers/ops.py, in several functions as following. It will lead to a mistake when run in python3 because locals_var.items() is of type dict_items, which can't be modified during iteration. But the code like "kwargs[name] = val" in the for loop will change locals() and lead to change in locals_var.items(). To solve this problem, just use locals_var = locals().copy() instead.

def hard_shrink(x, threshold=None):
locals_var = locals()
kwargs = dict()
for name, val in locals_var.items():
if val is not None:
kwargs[name] = val
return hard_shrink(**kwargs)