Created by: Aurelius84

• Fix the bug that NameVisitor parsed True/False as arguments.
• Support transform two independent function in dygraph_to_static_output.
• Polish the comment of function.

Here，if decorate many function by dygraph_to_static_output, the variables and ops will be added into the same main_program, which is consistent with static graph.

• The variables and ops in foo and bar will belong to the same main_program.

@dygraph_to_static_output
def foo(x, y):
      ....

@dygraph_to_static_output
def bar(x):
    ....

# run program
z = foo(x, y)
out = bar(z)

If want to split the two functions into different main_program, with program_guard is required, which is also consistent with static graph.

• The variables and ops in foo and bar will belong to the different main_program.

@dygraph_to_static_output
def foo(x, y):
      ....

@dygraph_to_static_output
def bar(x):
    ....

# run program
with program_guard(fluid.Program()):
    out = foo(x, y)

with program_guard(fluid.Program()):
    out = bar(x)