[caffe2] move the SaveOp implementation from a header to a .cc file
Summary:
Move the `SaveOp` code from `load_save_op.h` to `load_save_op.cc`.
Previously this implementation was all in the templatized `SaveOp` class, even
though most of the logic didn't depend on the template parameters. Having
this code be in the header file slows down the build, and forces more files to
be rebuilt than necessary when changing the SaveOp code. Having this code be
in a template class can also increase the generated code size be larger than
needed, as we don't need separate copies instantiated for each context type.
Test Plan: buck test //caffe2/caffe2/python/operator_test:load_save_test
Reviewed By: mraway
Differential Revision: D26641600
fbshipit-source-id: 84ebe8164ffac1e4a691be41147f0c5d8e890e09
