add WAR for pip>=19.3.1 (#652)

* add WAR for pip>=19.3.1

* remove pipmain, use extras_require instead

setup.py

@@ -2,7 +2,6 @@ import torch
 from setuptools import setup, find_packages
 import subprocess
 
-from pip._internal import main as pipmain
 import sys
 import warnings
 import os
@@ -31,10 +30,11 @@ if TORCH_MAJOR == 0 and TORCH_MINOR < 4:
 cmdclass = {}
 ext_modules = []
 
+extras = {}
 if "--pyprof" in sys.argv:
     with open('requirements.txt') as f:
         required_packages = f.read().splitlines()
-        pipmain(["install"] + required_packages)
+        extras['pyprof'] = required_packages
     try:
         sys.argv.remove("--pyprof")
     except:
@@ -209,4 +209,5 @@ setup(
     description='PyTorch Extensions written by NVIDIA',
     ext_modules=ext_modules,
     cmdclass=cmdclass,
+    extras_require=extras,
 )