Remove protobuf require and use requirements.txt (#10771)

Summary:
In prep for making FULL_CAFFE2 default, users shouldn't be required to have protobuf installed.

cc pjh5
Pull Request resolved: https://github.com/pytorch/pytorch/pull/10771

Reviewed By: pjh5

Differential Revision: D9474458

Pulled By: orionr

fbshipit-source-id: 3e28f5ce64d125a0a0418ce083f9ec73aec62492

requirements.txt

@@ -1,2 +1,6 @@
+future
+numpy
 pyyaml
+setuptools
+six
 typing

setup.py

@@ -1063,15 +1063,6 @@ cmdclass = {
 }
 cmdclass.update(build_dep_cmds)
 
-install_requires = [
-    'protobuf',
-    'pyyaml',
-    'numpy',
-    'future',
-    'setuptools',
-    'six',
-] if FULL_CAFFE2 else []
-
 entry_points = {}
 if FULL_CAFFE2:
     entry_points = {
@@ -1124,5 +1115,4 @@ if __name__ == '__main__':
                 'lib/include/torch/torch.h',
             ]
         },
-        install_requires=install_requires,
     )