Imperative fix conv2d's attrs bugs (#16158)

* Remove Desc in Forward Pass

* Refactor VarBase

* Add dbg info

* Only check type in imperative mode

* Polish code and support optimizer

test=develop

* Fix stop gradient problem in PyLayer

test=develop

* Fix conv2d's attrs

test=develop

python/paddle/fluid/imperative/nn.py

@@ -112,7 +112,7 @@ class Conv2D(layers.Layer):
                 'strides': self._stride,
                 'paddings': self._padding,
                 'dilations': self._dilation,
-                'groups': self._groups,
+                'groups': self._groups if self._groups else 1,
                 'use_cudnn': self._use_cudnn,
                 'use_mkldnn': False,
             })