Created by: Aurelius84

In dygraph, if tensor: is supported as followed when numel of tensor is 1:

if tensor:
    .....

# it is  equivalent to:
if tensor != 0:
    .....

In this PR, we fix the bug when node.if contains statement as above in IfConditionVisitor and support to transform if tensor into layers.cond.

if tensor:
    .....

# it will be transformed into:
bool_tensor_0 = fluid.layers.cast(x=tensor, dtype='bool')
res =  fluid.layers.cond(bool_tensor_0, lambda :true_fn_0(x), lambda : false_fn_0(x))