proper clean up after solving
bugfix: trainnet regularizer reset by testnet

Long story:

In Mocha.jl,  a shared Layer object is used in both the training and
testing network construction. The layer object contains a regularizer
object if the layer need regularization. The constructed network just
take that regularizer without copying. So the two networks turn out to
share the same regularizer. During network initialization, Mocha
re-scale all the layer regularization coefficient according to a global
factor.

The problem is: for the training network, the global regularization
coefficient comes from the solver, while the test net has no such
information, thus sticking to the default global coefficient 0. On the
other hand, since the two networks share the same regularizer object,
setting the coefficient of test net to 0 making the training net without
regularization, too. I was trapped in serious overfitting, and finally
found this bug after setting the regularization to 4000000 without
getting any differences…