[GUI] [bug] Fix "gui.set_image" not accept multi-channel scalar tenso…

…rs (#1264)

* [GUI] [bug] Fix "gui.set_image" not accept multi-channel scalar tensors

* rev

* [skip ci] fix doc link

docs/index.rst

@@ -62,7 +62,7 @@ The Taichi Programming Language
    :caption: Miscellaneous
    :maxdepth: 3
 
-   external_libraries
+   extension_libraries
    gui
    export_results
    cli_utilities

python/taichi/misc/gui.py

@@ -80,10 +80,11 @@ def set_image(self, img):
         import taichi as ti
 
         if isinstance(img, ti.Expr):
-            if ti.core.is_integral(img.data_type()):
-                # image of uint is not optimized by xxx_to_image
+            if ti.core.is_integral(img.data_type()) or len(img.shape()) != 2:
+                # Images of uint is not optimized by xxx_to_image
                 self.img = self.cook_image(img.to_numpy())
             else:
+                # Type matched! We can use an optimized copy kernel.
                 assert img.shape(
                 ) == self.res, "Image resolution does not match GUI resolution"
                 from taichi.lang.meta import tensor_to_image
@@ -94,6 +95,7 @@ def set_image(self, img):
             if ti.core.is_integral(img.data_type()):
                 self.img = self.cook_image(img.to_numpy())
             else:
+                # Type matched! We can use an optimized copy kernel.
                 assert img.shape(
                 ) == self.res, "Image resolution does not match GUI resolution"
                 assert img.n in [