最少的示例代碼：import theano as thimport theano.tensor as Timport numpy as npx = T.dscalars('x')z = th.shared(np.zeros(2))updates = [z, z+x]f1 = th.function(inputs=[x], updates=updates) f1(3)print z.get_value()錯誤信息：Traceback (most recent call last):  File "/home/temp2/theano.test.py", line 9, in <module>    f1 = th.function(inputs=[x], updates=updates)   File "/usr/local/lib/python2.7/dist-packages/theano/compile/function.py", line 205, in function    profile=profile)  File "/usr/local/lib/python2.7/dist-packages/theano/compile/pfunc.py", line 460, in pfunc    no_default_updates=no_default_updates)  File "/usr/local/lib/python2.7/dist-packages/theano/compile/pfunc.py", line 191, in rebuild_collect_shared    for (store_into, update_val) in iter_over_pairs(updates):  File "/usr/local/lib/python2.7/dist-packages/theano/tensor/basic.py", line 1610, in __iter__    for i in xrange(get_vector_length(self)):  File "/usr/local/lib/python2.7/dist-packages/theano/tensor/basic.py", line 5210, in get_vector_length    raise ValueError("length not known")ValueError: length not known此錯誤的原因是什么？