Add solver resumption (fix BVLC#6)

One can now specify trained_layers in the solver prototext and have the net simply continue training the layers with those predefined weights.

src/caffe/proto/caffe.proto

@@ -96,7 +96,7 @@ message NetParameter {
 // NOTE
 // Update the next available ID when you add a new SolverParameter field.
 //
-// SolverParameter next available ID: 37 (last added: iter_size)
+// SolverParameter next available ID: 38 (last added: trained_layers)
 message SolverParameter {
   //////////////////////////////////////////////////////////////////////////////
   // Specifying the train and test networks
@@ -115,6 +115,7 @@ message SolverParameter {
   // Proto filename for the train net, possibly combined with one or more
   // test nets.
   optional string net = 24;
+  optional string trained_layers = 37;
   // Inline train net param, possibly combined with one or more test nets.
   optional NetParameter net_param = 25;
 

src/caffe/solver.cpp

@@ -80,6 +80,10 @@ void Solver<Dtype>::InitTrainNet() {
   net_state.MergeFrom(param_.train_state());
   net_param.mutable_state()->CopyFrom(net_state);
   net_.reset(new Net<Dtype>(net_param));
+
+  if (param_.has_trained_layers()) {
+    net_->CopyTrainedLayersFrom(param_.trained_layers());
+  }
 }
 
 template <typename Dtype>