Add _field_to_tensors

src/pystencils_autodiff/__init__.py

 import pystencils_autodiff.backends
-# from pystencils_autodiff._field_to_tensors import (
-# tf_constant_from_field, tf_placeholder_from_field,
-# tf_scalar_variable_from_field, tf_variable_from_field,
-# torch_tensor_from_field)
+from pystencils_autodiff._field_to_tensors import (
+    tf_constant_from_field, tf_placeholder_from_field, tf_scalar_variable_from_field,
+    tf_variable_from_field, torch_tensor_from_field)
 from pystencils_autodiff.adjoint_field import AdjointField
-from pystencils_autodiff.autodiff import (AutoDiffAstPair, AutoDiffOp,
-                                          create_backward_assignments,
-                                          get_jacobian_of_assignments)
+from pystencils_autodiff.autodiff import (
+    AutoDiffAstPair, AutoDiffOp, create_backward_assignments, get_jacobian_of_assignments)
 
 __all__ = ['backends',
            'AdjointField',
@@ -14,7 +12,6 @@ __all__ = ['backends',
            'create_backward_assignments',
            'AutoDiffOp',
            'AutoDiffAstPair',
-           # "tf_constant_from_field", " tf_placeholder_from_field",
-           # "tf_scalar_variable_from_field", " tf_variable_from_field",
-           # "torch_tensor_from_field"
-           ]
+           "tf_constant_from_field", " tf_placeholder_from_field",
+           "tf_scalar_variable_from_field", " tf_variable_from_field",
+           "torch_tensor_from_field"]

src/pystencils_autodiff/_field_to_tensors.py

+import numpy as np
+
+try:
+    import tensorflow as tf
+except ImportError:
+    pass
+try:
+    import torch
+except ImportError:
+    pass
+
+
+def tf_constant_from_field(field, init_val=0):
+    return tf.constant(init_val, dtype=field.dtype.numpy_dtype, shape=field.shape, name=field.name + '_constant')
+
+
+def tf_scalar_variable_from_field(field, init_val, constraint=None):
+    var = tf.Variable(init_val, dtype=field.dtype.numpy_dtype, name=field.name + '_variable', constraint=constraint)
+    return var * tf_constant_from_field(field, 1)
+
+
+def tf_variable_from_field(field, init_val=0, constraint=None):
+    if isinstance(init_val, (int, float)):
+        init_val *= np.ones(field.shape, field.dtype.numpy_dtype)
+
+    return tf.Variable(init_val, dtype=field.dtype.numpy_dtype, name=field.name + '_variable', constraint=constraint)
+
+
+def tf_placeholder_from_field(field):
+    return tf.placeholder(dtype=field.dtype.numpy_dtype, name=field.name + '_placeholder', shape=field.shape)
+
+
+def torch_tensor_from_field(field, init_val=0, cuda=True, requires_grad=False):
+    if isinstance(init_val, (int, float)):
+        init_val *= np.ones(field.shape, field.dtype.numpy_dtype)
+    device = torch.device('cuda' if cuda else 'cpu')
+    return torch.tensor(init_val, requires_grad=requires_grad, device=device)