[4/5]Testing jit module in flatbuffer in Python. (pytorch#74387)

Summary:
Pull Request resolved: pytorch#74387

Make temporary python bindings for flatbuffer to test ScriptModule save / load.

(Note: this ignores all push blocking failures!)

Test Plan: unittest

Reviewed By: iseeyuan

Differential Revision: D34968080

fbshipit-source-id: d23b16abda6e4b7ecf6b1198ed6e00908a3db903
(cherry picked from commit 5cbbc39)

setup.py

@@ -824,7 +824,16 @@ def make_relative_rpath_args(path):
                   include_dirs=[],
                   library_dirs=library_dirs,
                   extra_link_args=extra_link_args + main_link_args + make_relative_rpath_args('lib'))
+    C_flatbuffer = Extension("torch._C_flatbuffer",
+                             libraries=main_libraries,
+                             sources=["torch/csrc/stub_with_flatbuffer.c"],
+                             language='c',
+                             extra_compile_args=main_compile_args + extra_compile_args,
+                             include_dirs=[],
+                             library_dirs=library_dirs,
+                             extra_link_args=extra_link_args + main_link_args + make_relative_rpath_args('lib'))
     extensions.append(C)
+    extensions.append(C_flatbuffer)
 
     if not IS_WINDOWS:
         DL = Extension("torch._dl",
@@ -932,6 +941,7 @@ def print_box(msg):
                 'bin/*',
                 'test/*',
                 '_C/*.pyi',
+                '_C_flatbuffer/*.pyi',
                 'cuda/*.pyi',
                 'optim/*.pyi',
                 'autograd/*.pyi',