Fixed a bug which caused that RNNs could not be used without variable

src/main/resources/templates/gluon/elements/GRU.ftl

 <#if element.member == "NONE">
 <#assign input = element.inputs[0]>
 <#if mode == "ARCHITECTURE_DEFINITION">
-            self.rnn_${element.element.name} = gluon.rnn.GRU(hidden_size=${element.units?c}, num_layers=${element.layers?c}, layout='NTC')
+            self.${element.name} = gluon.rnn.GRU(hidden_size=${element.units?c}, num_layers=${element.layers?c}, layout='NTC')
             <#include "OutputShape.ftl">
 <#elseif mode == "FORWARD_FUNCTION">
-        ${element.name}, ${element.element.name}_state_ = self.rnn_${element.element.name}(${input}, ${element.element.name}_state_)
+<#if element.isVariable()>
+        ${element.name}, ${element.element.name}_state_ = self.${element.name}(${input}, ${element.element.name}_state_)
+<#else>
+        ${element.name} = self.${element.name}(${input})
+</#if>
 </#if>
 <#elseif element.member == "STATE">
 <#if element.inputs?size gte 1>

src/main/resources/templates/gluon/elements/LSTM.ftl

 <#if element.member == "NONE">
 <#assign input = element.inputs[0]>
 <#if mode == "ARCHITECTURE_DEFINITION">
-            self.rnn_${element.element.name} = gluon.rnn.LSTM(hidden_size=${element.units?c}, num_layers=${element.layers?c}, layout='NTC')
+            self.${element.name} = gluon.rnn.LSTM(hidden_size=${element.units?c}, num_layers=${element.layers?c}, layout='NTC')
             <#include "OutputShape.ftl">
 <#elseif mode == "FORWARD_FUNCTION">
-        ${element.name}, ${element.element.name}_state_ = self.rnn_${element.element.name}(${input}, ${element.element.name}_state_)
+<#if element.isVariable()>
+        ${element.name}, ${element.element.name}_state_ = self.${element.name}(${input}, ${element.element.name}_state_)
+<#else>
+        ${element.name} = self.${element.name}(${input})
+</#if>
 </#if>
 <#elseif element.member == "STATE">
 <#if element.inputs?size gte 1>

src/main/resources/templates/gluon/elements/RNN.ftl

 <#if element.member == "NONE">
 <#assign input = element.inputs[0]>
 <#if mode == "ARCHITECTURE_DEFINITION">
-            self.rnn_${element.element.name} = gluon.rnn.RNN(hidden_size=${element.units?c}, num_layers=${element.layers?c}, activation='tanh', layout='NTC')
+            self.${element.name} = gluon.rnn.RNN(hidden_size=${element.units?c}, num_layers=${element.layers?c}, activation='tanh', layout='NTC')
             <#include "OutputShape.ftl">
 <#elseif mode == "FORWARD_FUNCTION">
-        ${element.name}, ${element.element.name}_state_ = self.rnn_${element.element.name}(${input}, ${element.element.name}_state_)
+<#if element.isVariable()>
+        ${element.name}, ${element.element.name}_state_ = self.${element.name}(${input}, ${element.element.name}_state_)
+<#else>
+        ${element.name} = self.${element.name}(${input})
+</#if>
 </#if>
 <#elseif element.member == "STATE">
 <#if element.inputs?size gte 1>