Workaround RTD building issue by using cpu version of TensorFlow

Using the hack from readthedocs/readthedocs.org#5512

Author: Yunqi Shao

.readthedocs.yml

@@ -11,6 +11,5 @@ formats: []
 python:
   version: 3.7
   install:
-    - requirements: doc/requirements.txt    
     - requirements: requirements-dev.txt
     - path: .

doc/requirements.txt

@@ -1,5 +1,12 @@
+import os
 from setuptools import setup, find_packages
 
+on_rtd = os.environ.get('READTHEDOCS') == 'True'
+if on_rtd:
+    INSTALL_REQUIRES = ['ase', 'numpy', 'pyyaml', 'tensorflow-cpu==1.15']
+else:
+    INSTALL_REQUIRES = ['ase', 'numpy', 'pyyaml', 'tensorflow==1.15']
+
 setup(name='pinn',
       version='dev',
       description='Pair interaction neural network',
@@ -8,7 +15,7 @@
       author_email='[email protected]',
       license='BSD',
       packages=find_packages(),
-      install_requires=['ase', 'numpy', 'pyyaml', 'tensorflow==1.15'],
+      install_requires=INSTALL_REQUIRES,
       entry_points={
           'console_scripts': ['pinn_train=pinn.trainer:main']
       }