make vision depend on pillow-simd if already installed (#522)

* make vision depend on pillow-simd if already installed

* actually make pillow-simd optional

setup.py

@@ -5,6 +5,7 @@ import re
 import shutil
 import sys
 from setuptools import setup, find_packages
+from pkg_resources import get_distribution, DistributionNotFound
 
 
 def read(*names, **kwargs):
@@ -15,6 +16,13 @@ def read(*names, **kwargs):
         return fp.read()
 
 
+def get_dist(pkgname):
+    try:
+        return get_distribution(pkgname)
+    except DistributionNotFound:
+        return None
+
+
 def find_version(*file_paths):
     version_file = read(*file_paths)
     version_match = re.search(r"^__version__ = ['\"]([^'\"]*)['\"]",
@@ -30,12 +38,15 @@ VERSION = find_version('torchvision', '__init__.py')
 
 requirements = [
     'numpy',
-    'pillow >= 4.1.1',
     'six',
     'torch',
     'tqdm'
 ]
 
+pillow_ver = ' >= 4.1.1'
+pillow_req = 'pillow-simd' if get_dist('pillow-simd') is not None else 'pillow'
+requirements.append(pillow_req + pillow_ver)
+
 setup(
     # Metadata
     name='torchvision',