Remove pure keyword from procedures

`differentiable_activation_strategy` and
`trainable_engine%to_inference_engine` because they contain
logic that is not allowed in pure procedures.

src/inference_engine/trainable_engine_m.f90

@@ -103,7 +103,7 @@ elemental module function num_layers(self) result(n_layers)
       integer n_layers
     end function
 
-    pure module function to_inference_engine(self) result(inference_engine)
+    module function to_inference_engine(self) result(inference_engine)
       implicit none
       class(trainable_engine_t), intent(in) :: self
       type(inference_engine_t) :: inference_engine

src/inference_engine/trainable_engine_s.f90

@@ -231,6 +231,9 @@
   end procedure
 
   module procedure to_inference_engine
+    ! assignment-stmt disallows the procedure from being pure because it might
+    ! deallocate polymorphic allocatable subcomponent `activation_strategy_`
+    ! TODO: consider how this affects design
     inference_engine = inference_engine_t(metadata = self%metadata_, weights = self%w, biases = self%b, nodes = self%n)
   end procedure
 

src/inference_engine/training_configuration_m.f90

@@ -74,7 +74,7 @@ elemental module function learning_rate(self) result(rate)
       real(rkind) rate
     end function
 
-    pure module function differentiable_activation_strategy(self) result(strategy)
+    module function differentiable_activation_strategy(self) result(strategy)
       implicit none
       class(training_configuration_t), intent(in) :: self
       class(differentiable_activation_strategy_t), allocatable :: strategy