Tensorflow error when saving params with probed weights

[hunse]

When trying to use get_nengo_params on a connection with probed weights, I get the error below. This only seems to happen after 6ddf68e.

Traceback (most recent call last):
  File "test_save_nengo_params.py", line 33, in <module>
    params = sim.get_nengo_params(conn, as_dict=False)
  File "/data/eric/workspace/nengo_dl/nengo_dl/simulator.py", line 892, in get_nengo_params
    data = self.data.get_params(*fetches)
  File "/data/eric/workspace/nengo_dl/nengo_dl/simulator.py", line 1488, in get_params
    fetches[placeholder] = self.sim.tensor_graph.get_tensor(sig)
  File "/data/eric/workspace/nengo_dl/nengo_dl/tensor_graph.py", line 29, in func_with_self
    return func(self, *args, **kwargs)
  File "/data/eric/workspace/nengo_dl/nengo_dl/tensor_graph.py", line 712, in get_tensor
    return tf.gather(base, tensor_sig.tf_indices)
  File "/home/eric/venv/full3/lib/python3.4/site-packages/tensorflow/python/ops/array_ops.py", line 2585, in gather
    params, indices, validate_indices=validate_indices, name=name)
  File "/home/eric/venv/full3/lib/python3.4/site-packages/tensorflow/python/ops/gen_array_ops.py", line 1864, in gather
    validate_indices=validate_indices, name=name)
  File "/home/eric/venv/full3/lib/python3.4/site-packages/tensorflow/python/framework/op_def_library.py", line 787, in _apply_op_helper
    op_def=op_def)
  File "/home/eric/venv/full3/lib/python3.4/site-packages/tensorflow/python/framework/ops.py", line 3160, in create_op
    op_def=op_def)
  File "/home/eric/venv/full3/lib/python3.4/site-packages/tensorflow/python/framework/ops.py", line 1672, in __init__
    control_flow_util.CheckInputFromValidContext(self, input_tensor.op)
  File "/home/eric/venv/full3/lib/python3.4/site-packages/tensorflow/python/ops/control_flow_util.py", line 200, in CheckInputFromValidContext
    raise ValueError(error_msg + " See info log for more details.")
ValueError: Cannot use 'while/iteration_0/Const_3' as input to 'Gather' because 'while/iteration_0/Const_3' is in a while loop. See info log for more details.

Here's a MWE:

import numpy as np
import nengo
import nengo_dl
from nengo_dl import tensor_layer


DO_ERROR = True

n_inputs = 3
n_neurons = 2

neuron_type = nengo_dl.SoftLIFRate()

# --- make network
with nengo.Network() as net:
    nengo_dl.configure_settings(trainable=None)
    net.config[nengo.Connection].synapse = None
    net.config[nengo.Ensemble].trainable = False

    inp = nengo.Node(np.zeros(n_inputs), label='input_node')
    layer, layer_conn = tensor_layer(inp, neuron_type,
                                     transform=nengo_dl.dists.Glorot(),
                                     shape_in=n_neurons,
                                     return_conn=True)

    if DO_ERROR:
        nengo.Probe(layer_conn, 'weights')

with nengo_dl.Simulator(net, minibatch_size=1) as sim:
    for conn in net.connections:
        print(conn)
        params = sim.get_nengo_params(conn, as_dict=False)

[drasmuss]

Should be fixed in #57, let me know if that works for you!