Adds pinv to tensorflow backend (https://github.com/google/TensorNetwork/issues/844)

tensornetwork/backends/tensorflow/tensorflow_backend.py

@@ -430,3 +430,26 @@ def eps(self, dtype: Type[np.number]) -> float:
       float: Machine epsilon.
     """
     return tf.experimental.numpy.finfo(dtype).eps
+
+  def pinv(self, tensor: Tensor, rcond: float = 1E-15, hermitian: bool = False) -> Tensor:
+    """
+    Compute the Moore-Penrose/Pseudo inverse of a tensor.
+
+    Args:
+      tensor: A tensor.
+      rcond: Cutoff for small singular values
+      hermitian: If True, matrix provided in function argument is assumed to be
+                  Hermitian (symmetric if real-valued)
+                  This argument is not supported in the TensorFlow
+                  backend and an error will be raised if it is
+                  specified.
+
+    Returns:
+      tensor: The pseudo inverse of `tensor`
+    """
+    if hermitian != False:
+      errstr = (f"hermitian = {hermitian} must be False (the default)"
+                f"with TensorFlow backend.")
+      raise NotImplementedError(errstr)
+
+    return tf.linalg.pinv(tensor, rcond=rcond)

tensornetwork/backends/tensorflow/tensorflow_backend_test.py

@@ -629,3 +629,15 @@ def test_power(dtype):
 def test_eps(dtype):
   backend = tensorflow_backend.TensorFlowBackend()
   assert backend.eps(dtype) == tf.experimental.numpy.finfo(dtype).eps
+
+def test_pinv():
+  backend = tensorflow_backend.TensorFlowBackend()
+  np.random.seed(10)
+  tensor = np.random.rand(2, 3, 4)
+  rcond = np.random.rand(1)[0] * 1E-14
+  expected = tf.linalg.pinv(tensor, rcond)
+  actual = backend.pinv(tensor, rcond)
+  np.testing.assert_allclose(expected, actual)
+  expected = tf.linalg.pinv(tensor)
+  actual = backend.pinv(tensor)
+  np.testing.assert_allclose(expected, actual)