Many programming languages provide `switch` as a generalization of `if-elif-else`. We want to add it to Fluid.
The following example shows the usage of `fluid.switch`.
```python
a=fluid.Var(10)
b=fluid.Var(0)
switch=fluid.switch()
withswitch.block():
withswitch.case(fluid.less_equal(a,10)):
fluid.print("Case 1")
withswitch.case(fluid.larger(a,0)):
fluid.print("Case 2")
withswitch.default():
fluid.print("Case 3")
```
### The Semantics
1. A `switch` control-flow checks cases one-by-one.
1. The condition of each case is a boolean value, which is a scalar, and differs from the `fluid.if_else` control-flow, which condition could be a vector of boolean values.
1. It runs the first matched case, or the default case if there is one.
1. Once it matches a case, it runs the corresponding branch and only that branch. It's like there is a C's `break` keyword at the end of each case.
The above program should print and print only "Case 1".
The implementation of the backward pass of the `switch` control-flow is easier than the backward of the `if_else`, because `switch` runs at most one branch, whereas `if-else` could run more than one branches.