Update _linear_operator.py (#9)

* Update _linear_operator.py

Add additional LinearOperator.__add__ case for Pythonic addition operations with the builtin sum() function. After other common use-cases, check if other is a numeric Python type and equal to zero, and if so, return self.

* Update linear_operator/operators/_linear_operator.py

Co-authored-by: Max Balandat <Balandat@users.noreply.github.com>

* Update _linear_operator.py

Co-authored-by: Geoff Pleiss <gpleiss@gmail.com>
Co-authored-by: Max Balandat <Balandat@users.noreply.github.com>

linear_operator/operators/_linear_operator.py

@@ -4,6 +4,7 @@
 
 import functools
 import math
+import numbers
 import warnings
 from abc import ABC, abstractmethod
 from copy import deepcopy
@@ -2546,9 +2547,7 @@ def zero_mean_mvn_samples(self, num_samples: int) -> torch.Tensor:
 
         return samples
 
-    def __add__(self, other: Union[torch.Tensor, LinearOperator, float]) -> LinearOperator:
-        from torch import Tensor
-
+    def __add__(self, other: Union[Tensor, LinearOperator, float]) -> LinearOperator:
         from .added_diag_linear_operator import AddedDiagLinearOperator
         from .dense_linear_operator import to_linear_operator
         from .diag_linear_operator import DiagLinearOperator
@@ -2568,6 +2567,8 @@ def __add__(self, other: Union[torch.Tensor, LinearOperator, float]) -> LinearOp
             new_self = self if self.shape[:-2] == shape[:-2] else self._expand_batch(shape[:-2])
             new_other = other if other.shape[:-2] == shape[:-2] else other._expand_batch(shape[:-2])
             return SumLinearOperator(new_self, new_other)
+        elif isinstance(other, numbers.Number) and other == 0:
+            return self
         else:
             return SumLinearOperator(self, other)