Merge branch 'master' into beta

Author: refraction-ray

CHANGELOG.md

@@ -22,6 +22,8 @@
 
 - Fix adjoint gate numpy conversion for fixed gate case
 
+- Sometime, tf just return IndexedSlice instead of tensor from gradient API, partially fix this in tc backend methods
+
 ### Changed
 
 - Upgraded black and mypy==1.2.0 (breaking change for developers)

tensorcircuit/backends/tensorflow_backend.py

@@ -725,6 +725,11 @@ def vjp(
         for i, gi in enumerate(g):
             if gi is None:
                 g[i] = tf.zeros_like(inputs[i])
+            if isinstance(gi, tf.IndexedSlices):
+                # gradient can return sth weird
+                # TODO(@refraction-ray): check whether other AD tf methods have such issues
+                # shape is still unkown, dense_shape attr doesn't work?
+                g[i] = tf.convert_to_tensor(gi)
         g = tuple(g)
         if one_input:
             g = g[0]