Allow installing only tf or torch

Author: gitanshu

setup.py

@@ -21,15 +21,21 @@
     'scikit-image',
     'scipy',
     'setproctitle>=1.0',
+]
+
+# Dependencies for optional features
+EXTRAS = {}
+
+EXTRAS['tensorflow'] = [
     'tensorflow>=1.14',
     'tensorflow-probability>=0.11.0',
+]
+
+EXTRAS['torch'] = [
     'torch>=1.0.0,!=1.5.0',
     'torchvision>=0.2.1',
 ]
 
-# Dependencies for optional features
-EXTRAS = {}
-
 EXTRAS['gym'] = [
     f'gym[atari,box2d,classic_control]=={GYM_VERSION}',
 ]