[pycaffe] expose solver update to do manual solving

a sketch of `solver.step()` done out manually:

1. `solver.net.forward()`
2. `solver.net.backward()`
3. `solver.net.apply_update()`
4. `solver.net.clear_param_diffs()`

Author: mitar

include/caffe/sgd_solvers.hpp

@@ -23,10 +23,11 @@ class SGDSolver : public Solver<Dtype> {
 
   const vector<shared_ptr<Blob<Dtype> > >& history() { return history_; }
 
+  virtual void ApplyUpdate();
+
  protected:
   void PreSolve();
   Dtype GetLearningRate();
-  virtual void ApplyUpdate();
   virtual void Normalize(int param_id);
   virtual void Regularize(int param_id);
   virtual void ComputeUpdateValue(int param_id, Dtype rate);

include/caffe/solver.hpp

@@ -94,9 +94,10 @@ class Solver {
    */
   virtual inline const char* type() const { return ""; }
 
- protected:
   // Make and apply the update value for the current iteration.
   virtual void ApplyUpdate() = 0;
+
+ protected:
   string SnapshotFilename(const string extension);
   string SnapshotToBinaryProto();
   string SnapshotToHDF5();

python/caffe/_caffe.cpp

@@ -507,6 +507,7 @@ BOOST_PYTHON_MODULE(_caffe) {
     .def("restore", &Solver<Dtype>::Restore)
     .def("snapshot", &Solver<Dtype>::Snapshot)
     .def("share_weights", &share_weights)
+    .def("apply_update", &Solver<Dtype>::ApplyUpdate)
     .add_property("param", bp::make_function(&Solver<Dtype>::param,
               bp::return_value_policy<bp::copy_const_reference>()));
   BP_REGISTER_SHARED_PTR_TO_PYTHON(Solver<Dtype>);