--enablesampling is broken

In chat:
--beamwidth=0
--enablesampling

This causes a traceback:

```
Traceback (most recent call last):
File "chat.py", line 43, in
model_dir = model_dir) as model:
File "C:\Source\Repos\seq2seq-chatbot\seq2seq-chatbot\chatbot_model.py", line 112, in __init__
self.predictions, self.predictions_seq_lengths = self._build_model()
File "C:\Source\Repos\seq2seq-chatbot\seq2seq-chatbot\chatbot_model.py", line 492, in _build_model
output_layer)
File "C:\Source\Repos\seq2seq-chatbot\seq2seq-chatbot\chatbot_model.py", line 731, in _build_inference_decoder
scope = decoder_scope)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\contrib\seq2seq\python\ops\decoder.py", line 323, in dynamic_decode
swap_memory=swap_memory)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\ops\control_flow_ops.py", line 3209, in while_loop
result = loop_context.BuildLoop(cond, body, loop_vars, shape_invariants)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\ops\control_flow_ops.py", line 2941, in BuildLoop
pred, body, original_loop_vars, loop_vars, shape_invariants)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\ops\control_flow_ops.py", line 2915, in _BuildLoop
next_vars.append(_AddNextAndBackEdge(m, v))
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\ops\control_flow_ops.py", line 661, in _AddNextAndBackEdge
_EnforceShapeInvariant(m, v)
File "C:\ProgramData\Anaconda3\lib\site-packages\tensorflow\python\ops\control_flow_ops.py", line 599, in _EnforceShapeInvariant
assert input_t.shape == m_shape
AssertionError
```