made tf an optional dependency

Author: davidsebfischer

setup.py

@@ -17,8 +17,6 @@
     long_description_content_type="text/markdown",
     packages=find_packages(),
     install_requires=[
-        'tensorflow>=1.14.0',
-        'tensorflow-probability>=0.7',
         'numpy>=1.16.4',
         'scipy>=1.2.1',
         'pandas',
@@ -27,6 +25,9 @@
         'patsy',
     ],
     extras_require={
+        'optional': [
+            'tensorflow>=1.14.0'
+        ],
         'scanpy_deps': [
             "scanpy",
             "anndata"