get Sphinx documentation to build pytorch NNCG optimizer

deepxde/optimizers/__init__.py

@@ -1,10 +1,22 @@
 import importlib
+import os
 import sys
 
 from .config import LBFGS_options, set_LBFGS_options, NNCG_options, set_NNCG_options
 from ..backend import backend_name
 
 
+# To get Sphinx documentation to build, we import all
+if os.environ.get("READTHEDOCS") == "True":
+    # The backend should be tensorflow/tensorflow.compat.v1 to ensure backend.tf is not
+    # None.
+    from . import jax
+    from . import paddle
+    from . import pytorch
+    from . import tensorflow
+    from . import tensorflow_compat_v1
+
+
 def _load_backend(mod_name):
     mod = importlib.import_module(".%s" % mod_name, __name__)
     thismod = sys.modules[__name__]